Is there an online code coloring service?

CodeColorizer is one.

Prettify is the code colorizer that Stack Overflow uses.

Also you can use http://tohtml.com/html/ or GeSHi

You may want to look at SyntaxHighligher. It uses JavaScript so it's not using your own server's resources and supports a bunch of color schemes. http://alexgorbatchev.com/wiki/SyntaxHighlighter

The problem with putting your code in a generator on another website is that if you change it somewhere you have to put it back in, which can become tedious.