If you draw two circles that are exactly the same in all aspects (location, size, etc...) except they have different colors, then the "border" of the underlying circle is showing through.
\documentclass[10pt]{book}
\usepackage{xcolor}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\fill (0,0) circle (0.1);
\fill[color=red] (0,0) circle (0.1);
\fill[color=red] (0.5,0) circle (0.1);
\end{tikzpicture}
\end{document}
Notice the first circle has a black "edge" around it that looks different than the 2nd circle which doesn't.
The problem is due to the fact that one is anti-aliasing both circles and the edge colors are being blended.
Is there a way to fix this but still have anti-aliasing? (basically render all the objects I need then anti-alias them properly afterwards?)
(I realize TikZ probably can't do anything about this but one doesn't know unless they ask)
TikZ
already loadsxcolor
so there is no need to do so yourself separately.