There are lots
of text editors that can do syntax highlighting and colorizing, and many are configurable. I agree with the suggestion of SciTE. SciTE is based on Neil Hodgson's Scintilla edit control, which includes code folding and syntax highlighting as part of what it does. As previously mentioned, Notepad++ and Notepad2 are also based on Scintilla, as are a number of other editors.
But whichever you choose, you'll still be faced with creating the rules the editor will use to do the coloring, and adding those rules to the existing lexer files the editor has. You probably want to spend some time thinking about just how you want your text to be colored, and under what circumstances, to have an idea of what your rules should be. Then you can look at some editors with configurable highlighting and see what would be involved in getting them to do what you want.
Stop by TextEditors.org
and poke around. It's a wiki devoted to text editors that is trying to be as comprehensive as possible. If it's a text editor, the site wants to list it. Mainframe, mini, Unix box, PC, PDA - whatever platform and OS. You can probably find some candidates to investigate. (There are enough Scintilla based editors to merit their own Family.)