How do I fill squares with particular colors in a grid?

Question

I currently have a 2x2 grid using this code:

\begin{center}
\begin{tikzpicture}
\draw (0,0) grid (2,2);

\end{tikzpicture}
\end{center}

I was wondering how I would be able to color for instance the top-left square black and the bottom-right square green.

Thanks!

Answer 1

To fill a rectangle, you only need to say

\fill[<color>] (<corner>) rectangle (<diagonally opposite corner>);

so the TikZ code for the question is as simple as

\fill (0,1) rectangle ++ (1,1); 
\fill[green] (1,0) rectangle ++ (1,1); 
\draw (0,0) grid (2,2); 

As you can see, we add three different paths. Note also that we draw the grid last.

Sometimes it might be worthwhile to have everything in one path macro. Then one can add the filled rectangles either via nodes or pics, among other things. If one does that, and wants the grid on top, one may want to use the behind path option. Here is a complete code with a few examples.

\documentclass[tikz,border=3mm]{standalone} 
\begin{document} 
\begin{tikzpicture} 
\begin{scope}[local bounding box=L]
 \fill (0,1) rectangle ++ (1,1); 
 \fill[green] (1,0) rectangle ++ (1,1); 
 \draw (0,0) grid (2,2); 
\end{scope} 
%
\begin{scope}[xshift=3cm,local bounding box=M,
    block/.style={minimum size=1cm,fill=#1,anchor=south west,inner sep=0pt,
    behind path}]
 \draw (0,1) node[block=black]{}
  (1,0) node[block=green]{}
 (0,0) grid (2,2); 
\end{scope} 
%
\begin{scope}[xshift=6cm,local bounding box=R,
    pics/block/.style={code={\path[pic actions] (0,0) rectangle ++ (1,1);}}]
 \draw[behind path] (0,1) pic[fill]{block}
  (1,0) pic[fill=green]{block}
 (0,0) grid (2,2); 
\end{scope} 
%
\path[nodes={text depth=0.25ex,above,font=\sffamily}] 
    (L.north) node{fill}
    (M.north) node{node}
    (R.north) node{pic};
\end{tikzpicture} 
\end{document}