Simplifying Filling grids

Question

To draw a filled grid, I used the first code (from this post).

I managed to reduce the code a little bit by using the second code, utilizing \foreach.

Is it possible to simplify it further by filling a group of 9 squares in the grid, for example coordinates from point 0,0 to point 3,3, to be filled with green colour, and so on.

\documentclass[12pt,a4paper]{article}
\usepackage{tikz}
\begin{document}
\tikzset{%
filrd/.style={%
fill=red},
filgr/.style={%
fill=green},
filyw/.style={%
fill=yellow},
filbl/.style={%
fill=blue},
}
%begin defining grid ==========================
\newcommand\sqw{1}
\tikzset{
pics/square/.default={\sqw},
pics/square/.style = {
code = {
\draw[pic actions, draw=none] (0,0) rectangle (#1,#1);
}}}%draw=none is to avoid displaying the little squares outline
\renewcommand\sqw{1.3}
%end defining grid ==========================
\begin{center}
\begin{tikzpicture}
\draw[step=\sqw] (\sqw,\sqw) grid (10*\sqw, 16*\sqw);
%col1
\pic[filgr] at (1*\sqw,15*\sqw) {square};
\pic[filgr] at (1*\sqw,14*\sqw) {square};
\pic[filgr] at (1*\sqw,13*\sqw) {square};
\pic[filyw] at (1*\sqw,12*\sqw) {square};
\pic[filyw] at (1*\sqw,11*\sqw) {square};
\pic[filyw] at (1*\sqw,10*\sqw) {square};
\pic[filbl] at (1*\sqw,9*\sqw) {square};
\pic[filbl] at (1*\sqw,8*\sqw) {square};
\pic[filbl] at (1*\sqw,7*\sqw) {square};
\pic[filgr] at (1*\sqw,6*\sqw) {square};
\pic[filgr] at (1*\sqw,5*\sqw) {square};
\pic[filgr] at (1*\sqw,4*\sqw) {square};
\pic[filrd] at (1*\sqw,3*\sqw) {square};
\pic[filrd] at (1*\sqw,2*\sqw) {square};
\pic[filrd] at (1*\sqw,1*\sqw) {square};
%col2
\pic[filgr] at (2*\sqw,15*\sqw) {square};
\pic[filgr] at (2*\sqw,14*\sqw) {square};
\pic[filgr] at (2*\sqw,13*\sqw) {square};
\pic[filyw] at (2*\sqw,12*\sqw) {square};
\pic[filyw] at (2*\sqw,11*\sqw) {square};
\pic[filyw] at (2*\sqw,10*\sqw) {square};
\pic[filbl] at (2*\sqw,9*\sqw) {square};
\pic[filbl] at (2*\sqw,8*\sqw) {square};
\pic[filbl] at (2*\sqw,7*\sqw) {square};
\pic[filgr] at (2*\sqw,6*\sqw) {square};
\pic[filgr] at (2*\sqw,5*\sqw) {square};
\pic[filgr] at (2*\sqw,4*\sqw) {square};
\pic[filrd] at (2*\sqw,3*\sqw) {square};
\pic[filrd] at (2*\sqw,2*\sqw) {square};
\pic[filrd] at (2*\sqw,1*\sqw) {square};
%col3
\pic[filgr] at (3*\sqw,15*\sqw) {square};
\pic[filgr] at (3*\sqw,14*\sqw) {square};
\pic[filgr] at (3*\sqw,13*\sqw) {square};
\pic[filyw] at (3*\sqw,12*\sqw) {square};
\pic[filyw] at (3*\sqw,11*\sqw) {square};
\pic[filyw] at (3*\sqw,10*\sqw) {square};
\pic[filbl] at (3*\sqw,9*\sqw) {square};
\pic[filbl] at (3*\sqw,8*\sqw) {square};
\pic[filbl] at (3*\sqw,7*\sqw) {square};
\pic[filgr] at (3*\sqw,6*\sqw) {square};
\pic[filgr] at (3*\sqw,5*\sqw) {square};
\pic[filgr] at (3*\sqw,4*\sqw) {square};
\pic[filrd] at (3*\sqw,3*\sqw) {square};
\pic[filrd] at (3*\sqw,2*\sqw) {square};
\pic[filrd] at (3*\sqw,1*\sqw) {square};
%col4
\pic[filrd] at (4*\sqw,15*\sqw) {square};
\pic[filrd] at (4*\sqw,14*\sqw) {square};
\pic[filrd] at (4*\sqw,13*\sqw) {square};
\pic[filbl] at (4*\sqw,12*\sqw) {square};
\pic[filbl] at (4*\sqw,11*\sqw) {square};
\pic[filbl] at (4*\sqw,10*\sqw) {square};
\pic[filyw] at (4*\sqw,9*\sqw) {square};
\pic[filyw] at (4*\sqw,8*\sqw) {square};
\pic[filyw] at (4*\sqw,7*\sqw) {square};
\pic[filbl] at (4*\sqw,6*\sqw) {square};
\pic[filbl] at (4*\sqw,5*\sqw) {square};
\pic[filbl] at (4*\sqw,4*\sqw) {square};
\pic[filgr] at (4*\sqw,3*\sqw) {square};
\pic[filgr] at (4*\sqw,2*\sqw) {square};
\pic[filgr] at (4*\sqw,1*\sqw) {square};
%col5
\pic[filrd] at (5*\sqw,15*\sqw) {square};
\pic[filrd] at (5*\sqw,14*\sqw) {square};
\pic[filrd] at (5*\sqw,13*\sqw) {square};
\pic[filbl] at (5*\sqw,12*\sqw) {square};
\pic[filbl] at (5*\sqw,11*\sqw) {square};
\pic[filbl] at (5*\sqw,10*\sqw) {square};
\pic[filyw] at (5*\sqw,9*\sqw) {square};
\pic[filyw] at (5*\sqw,8*\sqw) {square};
\pic[filyw] at (5*\sqw,7*\sqw) {square};
\pic[filbl] at (5*\sqw,6*\sqw) {square};
\pic[filbl] at (5*\sqw,5*\sqw) {square};
\pic[filbl] at (5*\sqw,4*\sqw) {square};
\pic[filgr] at (5*\sqw,3*\sqw) {square};
\pic[filgr] at (5*\sqw,2*\sqw) {square};
\pic[filgr] at (5*\sqw,1*\sqw) {square};
%col6
\pic[filrd] at (6*\sqw,15*\sqw) {square};
\pic[filrd] at (6*\sqw,14*\sqw) {square};
\pic[filrd] at (6*\sqw,13*\sqw) {square};
\pic[filbl] at (6*\sqw,12*\sqw) {square};
\pic[filbl] at (6*\sqw,11*\sqw) {square};
\pic[filbl] at (6*\sqw,10*\sqw) {square};
\pic[filyw] at (6*\sqw,9*\sqw) {square};
\pic[filyw] at (6*\sqw,8*\sqw) {square};
\pic[filyw] at (6*\sqw,7*\sqw) {square};
\pic[filbl] at (6*\sqw,6*\sqw) {square};
\pic[filbl] at (6*\sqw,5*\sqw) {square};
\pic[filbl] at (6*\sqw,4*\sqw) {square};
\pic[filgr] at (6*\sqw,3*\sqw) {square};
\pic[filgr] at (6*\sqw,2*\sqw) {square};
\pic[filgr] at (6*\sqw,1*\sqw) {square};
%col7
\pic[filyw] at (7*\sqw,15*\sqw) {square};
\pic[filyw] at (7*\sqw,14*\sqw) {square};
\pic[filyw] at (7*\sqw,13*\sqw) {square};
\pic[filgr] at (7*\sqw,12*\sqw) {square};
\pic[filgr] at (7*\sqw,11*\sqw) {square};
\pic[filgr] at (7*\sqw,10*\sqw) {square};
\pic[filrd] at (7*\sqw,9*\sqw) {square};
\pic[filrd] at (7*\sqw,8*\sqw) {square};
\pic[filrd] at (7*\sqw,7*\sqw) {square};
\pic[filyw] at (7*\sqw,6*\sqw) {square};
\pic[filyw] at (7*\sqw,5*\sqw) {square};
\pic[filyw] at (7*\sqw,4*\sqw) {square};
\pic[filbl] at (7*\sqw,3*\sqw) {square};
\pic[filbl] at (7*\sqw,2*\sqw) {square};
\pic[filbl] at (7*\sqw,1*\sqw) {square};
%col8
\pic[filyw] at (8*\sqw,15*\sqw) {square};
\pic[filyw] at (8*\sqw,14*\sqw) {square};
\pic[filyw] at (8*\sqw,13*\sqw) {square};
\pic[filgr] at (8*\sqw,12*\sqw) {square};
\pic[filgr] at (8*\sqw,11*\sqw) {square};
\pic[filgr] at (8*\sqw,10*\sqw) {square};
\pic[filrd] at (8*\sqw,9*\sqw) {square};
\pic[filrd] at (8*\sqw,8*\sqw) {square};
\pic[filrd] at (8*\sqw,7*\sqw) {square};
\pic[filyw] at (8*\sqw,6*\sqw) {square};
\pic[filyw] at (8*\sqw,5*\sqw) {square};
\pic[filyw] at (8*\sqw,4*\sqw) {square};
\pic[filbl] at (8*\sqw,3*\sqw) {square};
\pic[filbl] at (8*\sqw,2*\sqw) {square};
\pic[filbl] at (8*\sqw,1*\sqw) {square};
%col9
\pic[filyw] at (9*\sqw,15*\sqw) {square};
\pic[filyw] at (9*\sqw,14*\sqw) {square};
\pic[filyw] at (9*\sqw,13*\sqw) {square};
\pic[filgr] at (9*\sqw,12*\sqw) {square};
\pic[filgr] at (9*\sqw,11*\sqw) {square};
\pic[filgr] at (9*\sqw,10*\sqw) {square};
\pic[filrd] at (9*\sqw,9*\sqw) {square};
\pic[filrd] at (9*\sqw,8*\sqw) {square};
\pic[filrd] at (9*\sqw,7*\sqw) {square};
\pic[filyw] at (9*\sqw,6*\sqw) {square};
\pic[filyw] at (9*\sqw,5*\sqw) {square};
\pic[filyw] at (9*\sqw,4*\sqw) {square};
\pic[filbl] at (9*\sqw,3*\sqw) {square};
\pic[filbl] at (9*\sqw,2*\sqw) {square};
\pic[filbl] at (9*\sqw,1*\sqw) {square};
\end{tikzpicture}
\end{center}

\begin{center}
\begin{tikzpicture}
\draw[step=\sqw] (\sqw,\sqw) grid (10*\sqw, 16*\sqw);
%gr
\foreach \i in {15,14,13,6,5,4}{ 
\foreach \j in {1,2,3}{ 
\pic[filgr] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {18,17,16,3,2,1}{ 
\foreach \j in {4,5,6}{ 
\pic[filgr] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {21,20,19,12,11,10}{ 
\foreach \j in {7,8,9}{ 
\pic[filgr] at (\j*\sqw,\i*\sqw) {square};};};
%yw
\foreach \i in {21,20,19,12,11,10}{ 
\foreach \j in {1,2,3}{ 
\pic[filyw] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {9,8,7}{ 
\foreach \j in {4,5,6}{ 
\pic[filyw] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {15,14,13,6,5,4}{ 
\foreach \j in {7,8,9}{ 
\pic[filyw] at (\j*\sqw,\i*\sqw) {square};};};
%bl
\foreach \i in {18,17,16,9,8,7}{ 
\foreach \j in {1,2,3}{ 
\pic[filbl] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {21,20,19,12,11,10,6,5,4}{ 
\foreach \j in {4,5,6}{ 
\pic[filbl] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {3,2,1}{ 
\foreach \j in {7,8,9}{ 
\pic[filbl] at (\j*\sqw,\i*\sqw) {square};};};
%rd
\foreach \i in {3,2,1}{ 
\foreach \j in {1,2,3}{ 
\pic[filrd] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {15,14,13}{ 
\foreach \j in {4,5,6}{ 
\pic[filrd] at (\j*\sqw,\i*\sqw) {square};};};
%
\foreach \i in {18,17,16,9,8,7}{ 
\foreach \j in {7,8,9}{ 
\pic[filrd] at (\j*\sqw,\i*\sqw) {square};};};
%
\end{tikzpicture}
\end{center}
\end{document}

Answer 1

I don't know your goal, so I don't know either if this is the right approach. It's very simple, though. It only needs a \foreach that loops over the colors. Then I apply the dimension \sqw as a scale factor instead of applying it in each element. I suppose, seen the link you provided, that you need more drawings over this grid. If the scale is not appropriate for them just put all my code inside a scope and apply the scale to this scope.

For example:

\documentclass[tikz,border=2mm]{standalone}

\definecolor{colorg}{HTML}{00FF00} % green
\definecolor{colorr}{HTML}{FF0000} % red
\definecolor{colory}{HTML}{FFFF00} % yellow
\definecolor{colorb}{HTML}{0000FF} % blue

\begin{document}
\newcommand\sqw{1}
\begin{tikzpicture}[scale=\sqw,y=-1cm]
\foreach[count=\ii]\i in {g,r,y,
                          y,b,g,
                          b,y,r,
                          g,b,y,
                          r,g,b}
{
  \pgfmathsetmacro\myrow{3*mod(\ii-1,3)}
  \pgfmathsetmacro\mycol{3*div(\ii-1,3)}
  \fill[color\i] (\myrow,\mycol) rectangle ++ (3,3);
}
\draw (0,0) grid (9,15);
\end{tikzpicture}
\end{document}

Edit: This is what I had in mind, but as I said, I don't know if it is the better approach:

\documentclass[tikz,border=2mm]{standalone}

\definecolor{colorg}{HTML}{00FF00} % green
\definecolor{colorr}{HTML}{FF0000} % red
\definecolor{colory}{HTML}{FFFF00} % yellow
\definecolor{colorb}{HTML}{0000FF} % blue
\definecolor{colorw}{HTML}{FFFFFF} % white

\begin{document}
\newcommand\sqw{1}
\begin{tikzpicture}[scale=\sqw,y=-1cm]
\foreach[count=\ii]\i in {g,r,y,
                          y,b,g,
                          b,y,r,
                          g,b,y,
                          r,g,b,
                          w,w,y}
{
  \pgfmathsetmacro\myrow{3*mod(\ii-1,3)}
  \pgfmathsetmacro\mycol{3*div(\ii-1,3)}
  \fill[color\i] (\myrow,\mycol) rectangle ++ (3,3);
}
\draw (0,0) grid (9,18);

\foreach\x/\y in {0/15,1/17,2/17} 
  \draw[very thick,fill=magenta] (\x+0.5,\y+0.5) circle [radius=0.3cm]; % <-- here you can define a \pic
\foreach\x/\y in {3/15,4/16,4/17,3/17}
  \draw[very thick,fill=brown] (\x+0.2,\y+0.2) rectangle ++ (0.6,0.6);  % <-- here you can define another \pic
\end{tikzpicture}
\end{document}