How to draw a solid colored circle?

One easy way would be to use TikZ as in the following MWE

\documentclass[a4paper,12pt]{scrartcl}
\usepackage{tikz}

\begin{document}
Some Text \tikz\draw[red,fill=red] (0,0) circle (.5ex); further text
\end{document}

which produces

Where the first red defines the line style of the drawn circle to be red and the fill=red specifies, that its solid red. You could also use black,fill=red to obtain a red circle with a black border. Finally of course the .5ex is the radius of the circle.

Another solution with TikZ, but this one creates a command \tikzcircle to be used in the document:

\newcommand{\tikzcircle}[2][red,fill=red]{\tikz[baseline=-0.5ex]\draw[#1,radius=#2] (0,0) circle ;}%

It takes one mandatory argument, the radius of the circle and an optional argument that helps in customizing the circle's aspect.

The code:

\documentclass[a4paper,11pt]{article}
\usepackage{tikz}

\newcommand{\tikzcircle}[2][red,fill=red]{\tikz[baseline=-0.5ex]\draw[#1,radius=#2] (0,0) circle ;}%

\begin{document}
This is my text \tikzcircle{2pt} followed by \tikzcircle[green, fill=blue]{1.5pt} some other text \tikzcircle[fill=orange]{3pt} and some other text
\end{document}

The result:

\documentclass{article}
\usepackage{xcolor,pict2e}% to allow any radius
\begin{document}
\leavevmode
\put(0,0){\circle{20.6}}\put(0,0){\color{red}\circle*{20}}

\end{document}