A possible solution (if I understand your question correct) is to direct a \color{red} command into all columns. I have defined a new column type R for the purpose. Thereafter, you enclose the heading text in \multicolumn{1}{c|}{<text>} to protect against colourisation.
You may adapt David Carlisle answer to this question to your code, and define a new column that accepts a \rowfont-command (also defined in the code):
Example 1 – defining a \rowfont-command
\documentclass[11pt,a4paper]{report}\usepackage[utf8]{inputenc}\usepackage{array}\newcolumntype{C}{>{\rowfont}c}\newcommand\setrowfont[1]{\noalign{\gdef\rowfont{#1}}}\gdef\rowfont{}\usepackage[left=1.00cm, right=1.00cm, top=1.00cm, bottom=1.00cm]{geometry}\usepackage{colortbl}\usepackage{chemformula}\begin{document} \begin{center} \footnotesize \renewcommand{\arraystretch}{1.5} \hspace*{-1cm}\begin{tabular}{|*{3}{C|}|*{8}{C|}} \hline \rowcolor{black} \multicolumn{11}{|c|}{\textcolor{white}{Corps purs}}\\\hline \rowcolor{gray!90}élémentaires & \multicolumn{2}{c||}{simples} & \multicolumn{8}{c|}{composés}\\ \hline \rowcolor{gray!70}& métal. & non-métal. & \multicolumn{2}{c|}{Oxydes} & Hydroxydes & \multicolumn{2}{c|}{Acides} & \multicolumn{2}{c|}{Sels} & Corps organiques\\ \hline \rowcolor{gray!50}&& & métal. & non-métal. && bin. & tern. & bin. & tern. &\\ \hline \setrowfont{\color{red}}&& X& & & & & & & &\ch{C6H12O6}\\ \hline && \ch{Fe2O3} & & & & & & & &\\ \hline X&X& X & X & X & X & X & X & X & X &X\\ \hline&& X& & & & & & & &\\ \hline&& X& & & & & & & &\\ \hline&& X& & & & & & & &\\ \hline \end{tabular} \end{center}\end{document}Example 2 –>{color{red} and\multicolomn}}
\documentclass[11pt,a4paper]{report}\usepackage[utf8]{inputenc}\usepackage{array}\usepackage[left=1.00cm, right=1.00cm, top=1.00cm, bottom=1.00cm]{geometry}\newcolumntype{R}{>{\color{red}}c}\usepackage{colortbl}\usepackage{chemformula}\begin{document} \begin{center} \footnotesize \renewcommand{\arraystretch}{1.5} \hspace*{-1cm}\begin{tabular}{|*{3}{R|}|*{8}{R|}} \hline \rowcolor{black} \multicolumn{11}{|c|}{\textcolor{white}{Corps purs}}\\\hline \rowcolor{gray!90} \multicolumn{1}{c|}{élémentaires} & \multicolumn{2}{c||}{simples} & \multicolumn{8}{c|}{composés}\\ \hline \rowcolor{gray!70}& \multicolumn{1}{c|}{métal.} & \multicolumn{1}{c||}{non-métal.} & \multicolumn{2}{c|}{Oxydes} & \multicolumn{1}{c|}{Hydroxydes} & \multicolumn{2}{c|}{Acides} & \multicolumn{2}{c|}{Sels} & \multicolumn{1}{c|}{Corps organiques}\\ \hline \rowcolor{gray!50}&& & \multicolumn{1}{c|}{métal.} & \multicolumn{1}{c|}{non-métal.} && \multicolumn{1}{c|}{bin.} & \multicolumn{1}{c|}{tern.} & \multicolumn{1}{c|}{bin.} & \multicolumn{1}{c|}{tern.} &\\ \hline && X& & & & & & & &\ch{C6H12O6}\\ \hline && \ch{Fe2O3} & & & & & & & &\\ \hline X&X& X & X & X & X & X & X & X & X &X\\ \hline&& X& & & & & & & &\\ \hline&& X& & & & & & & &\\ \hline&& X& & & & & & & &\\ \hline \end{tabular} \end{center}\end{document}