The expansion of the optional argument to \addplot was definitely quite finicky. In the end, I use a listofitems list to define and expand the colors. See ADDENDUM for version without listofitems.
\documentclass{article}\usepackage{tikz,pgfplots,pgffor,listofitems}\pgfplotsset{compat=1.17}\readlist\colorlist{green,yellow,orange,red}\begin{document} \begin{tikzpicture} \begin{axis}[ xtick={1,...,4}, x tick style={draw=none}, xticklabel={1}, ymin=0, every axis plot/.append style={ybar,fill} ] \readlist\mylist{1,3,2,10} \foreachitem \x \in \mylist[]{ \typeout{index is \xcnt} \edef\tmp{color=\colorlist[\xcnt]} \expandafter\addplot\expandafter[\tmp] coordinates {(\xcnt,\x)}; } \end{axis} \end{tikzpicture}\tikz[x=0.75cm,y=0.75cm]\foreach \x [count=\xi] in {a,...,d}\foreach \y [count=\yi] in {\x,...,d}\node [draw, top color=white, bottom color={\colorlist[\xi]}, minimum size=0.666cm]at (\xi,-\yi) {$\xi,\yi$};\end{document}
ADDENDUM
Version without using listofitems. In both cases, the key is expanding the color before presenting it to the \addplot, by way of
\edef\tmp{color=color@\n} \expandafter\addplot\expandafter[\tmp] coordinates {(\n,\x)};The MWE:
\documentclass{article}\usepackage{tikz,pgfplots,pgffor}\colorlet{color@1}{green}\colorlet{color@2}{yellow}\colorlet{color@3}{orange}\colorlet{color@4}{red}\begin{document} \begin{tikzpicture} \begin{axis}[ xtick={1,...,4}, x tick style={draw=none}, xticklabel={1}, ymin=0, every axis plot/.append style={ybar,fill} ] \foreach \x [count=\n] in {1,3,2,10}{ \typeout{index is \n} \edef\tmp{color=color@\n} \expandafter\addplot\expandafter[\tmp] coordinates {(\n,\x)}; } \end{axis} \end{tikzpicture}\tikz[x=0.75cm,y=0.75cm]\foreach \x [count=\xi] in {a,...,d}\foreach \y [count=\yi] in {\x,...,d}\node [draw, top color=white, bottom color=color@\xi, minimum size=0.666cm]at (\xi,-\yi) {$\xi,\yi$};\end{document}