Since you're using a range in \foreach
, I'm presuming that the number of circles is flexible; therefore, the number of colours involved should also be flexible.
Here is how I would do it.
- Make the loop variable (called
\n
, below) vary over a range of integers (from 1 to the total number of circles to be drawn).
- Use the
\foreach
's evaluate
key to generate the value of each radius (called \rr
below) on the basis of that loop variable.
- Use
\foreach
's evaluate
key to generate a value between 0 and 100 (called \sat
below) and use that value to change the circles' colour linearly between two colours (red and blue, for instance).

\documentclass{article}
\usepackage[paperheight=30cm,paperwidth=35cm,margin=1in,heightrounded]{geometry}
\usepackage{tikz}
\begin{document}
\fbox{\begin{tikzpicture}[scale=0.8] %
% center
\path (0,0) coordinate(A);
% circles
\def\Rfactor{0.5} % multiplying factor
\pgfmathtruncatemacro\nbcircles{9} % number of circles to draw
\foreach[
var=\n,
evaluate=\n as \rr using \Rfactor*(\n+1),
evaluate=\n as \sat using 100/(\nbcircles-1)*(\n-1),
] \rr in {1,...,\nbcircles}
{
\draw[blue!\sat!red] (A) circle (\rr);
}
\end{tikzpicture}}
\end{document}