Setting background color in flowchart

Question

I've a flowchart created in the following manner

\tikzstyle{steps} = [rectangle, rounded corners, minimum width=3cm, minimum height=1cm, text centered, text width=3cm, draw=black, fill=gray!50]
    \tikzstyle{process} = [rectangle, minimum width=3cm, minimum height=2cm, text centered, text width=4cm, draw=black, fill=white]
    \definecolor{arrowblue}{RGB}{98,145,224}



\newcommand\ImageNode[3][]{
    \node[draw=arrowblue!80!black,linewidth=1pt,#1](#2){\includegraphics[width=1.5cm,height=1.5cm]{#3}};
}

\begin{tikzpicture}[node distance=2cm]
\node (step1) [steps] {Step 1};
\node (step2) [steps, right of=step1, xshift=3cm]{Step 2};
\node (step3) [steps, right of=step2, xshift=3cm]{Step 3};
\node (process1) [process, below of=step1] {text text};
\node (process2) [process, right of=process1, xshift=3cm] {text text
    text text text text text text text text text text text text text text text text};
\node (process3) [process, right of=process2, xshift=3cm] {text text text};
\end{tikzpicture}

I'd like to add background color in gradients of red for step1 , step 2 and step 3. I am not sure how to do this because step1, step2, and step3 aren't specified as columns in the current setting.

Any suggestions on how to do this will b really helpful.

EDIT: I am looking for a way to create columns and fill the background color of nodes in a given column. I don't want to color the nodes. I want the boxes in step1 to be in column1, step2-column2 and step3-column3

EDIT2: I find additional boxes around the flowchart, I am not sure why this occurs. Any suggestions on how to remove these lines will be really helpful.

EDIT 3: I have added decision/.style = {diamond, minimum height=1cm , draw=black, fill=white, inner sep=2mm}, to the solution provided below

The column heights are uneven. I'd couldn't find an option to set maximum height to styles. Suggestions on how to set uniform column heights will be really helpful.

Answer 1

Like this?

\documentclass[tikz, margin=3mm]{standalone}
\usetikzlibrary{backgrounds,
                fit,
                positioning}

\begin{document} 
    \begin{tikzpicture}[
 node distance = 4mm and 4mm,
   base/.style = {rectangle, draw, align=center}, 
  steps/.style = {base, rounded corners, fill=gray!50, 
                  minimum height=1cm, text width=3cm},
process/.style = {base,
                  minimum height=2cm, text width=4cm, fill=white}
                  ]
\node (process1) [process]                      {text text};
\node (process2) [process, right=of process1]   {text text
    text text text text text text text text text text text text text text text text};
\node (process3) [process, right=of process2]   {text text text};
\node (step1) [steps, above=of process1]                   {Step 1};
\node (step2) [steps, above=of process2]   {Step 2};
\node (step3) [steps, above=of process3]   {Step 3};
%
\scoped[on background layer]
{
\node[fit=(step1) (process1), fill=red!20] {};
\node[fit=(step2) (process2), fill=red!40] {};
\node[fit=(step3) (process3), fill=red!60] {};
}
    \end{tikzpicture}
\end{document}

Adendum: If you use article document class with 12 pt font size, you need to make the following changes in the above proposed solution: replace \documentclass[tikz, margin=3mm]{standalone} with

\documentclass[12pt]{article}
\usepackage{tikz}

and reduce width of nodes (since otherwise protrude out of the right text border).

At this changes you may slightly improve image code as is done in the following MWE:

\documentclass[12pt]{article}   % changed
\usepackage{tikz}               % added
\usetikzlibrary{backgrounds,
                fit,
                positioning}

\begin{document} 
\begin{figure}[ht]
\centering
    \begin{tikzpicture}[
  node distance = 4mm and 5mm,
    base/.style = {rectangle, draw, inner sep=2mm, align=center}, 
   steps/.style = {base, rounded corners, fill=gray!50, 
                   minimum height=1cm, text width=24mm},    % changed
 process/.style = {base,
                   minimum height=3cm, text width=34mm,     % changed 
                   fill=white},
FIT/.style args = {#1/#2}{base, draw=none, fit=#1, fill=#2} % added
                  ]
\node (process1) [process]                      {text text};
\node (process2) [process, right=of process1]   {text text
    text text text text text text text text text text text text text text text text};
\node (process3) [process, right=of process2]   {text text text};
\node (step1) [steps, above=of process1]                   {Step 1};
\node (step2) [steps, above=of process2]   {Step 2};
\node (step3) [steps, above=of process3]   {Step 3};
%
\scoped[on background layer]
{
\node[FIT=(step1) (process1)/red!20] {};
\node[FIT=(step2) (process2)/red!40] {};
\node[FIT=(step3) (process3)/red!60] {};
}
    \end{tikzpicture}
\end{figure}
\end{document}

Result is now (to my opinion) a bit nicer :-) :

Answer 2

Background colour is done by the fill= option. Take it out of the process style, and put it in each node separately.

MWE

\documentclass{article}
\usepackage{tikz}

\tikzstyle{steps} = [rectangle, rounded corners, minimum width=3cm, minimum height=1cm, text centered, text width=3cm, draw=black, fill=gray!50]
    \tikzstyle{process} = [rectangle, minimum width=3cm, minimum height=2cm, text centered, text width=4cm, draw=black]
    \definecolor{arrowblue}{RGB}{98,145,224}



\newcommand\ImageNode[3][]{
    \node[draw=arrowblue!80!black,linewidth=1pt,#1](#2){\includegraphics[width=1.5cm,height=1.5cm]{#3}};
}

\begin{document}
\begin{tikzpicture}[node distance=2cm]
\node (step1) [steps] {Step 1};
\node (step2) [steps, right of=step1, xshift=3cm]{Step 2};
\node (step3) [steps, right of=step2, xshift=3cm]{Step 3};
\node (process1) [fill=red!10, process, below of=step1] {text text};
\node (process2) [fill=red!30, process, right of=process1, xshift=3cm] {text text
    text text text text text text text text text text text text text text text text};
\node (process3) [fill=red!90, process, right of=process2, xshift=3cm] {text text text};
\end{tikzpicture}
\end{document}