Modify Syntax Highlighting (e.g., teach TextMate new keywords in your language of choice).
For example, to cause TM to highlight 'eval' and 'compile' in Python:
(i) select 'Bundles' > 'Bundle Editor > 'Edit Languages';
(ii) choose 'Python' from the Bundles on the LHS of the Bundle Editor;
(iii) click on 'Python' so that you see the drop-down choices (preceded by 'L');
(iv) click the choice 'Python';
(v) make sure 'Editing' is toggled in the upper right-hand-cornder of the B/E;
(vi) on the RHS, you'll see the entire grammar for the selected language;
(vii) scroll down about 100 lines, until you see, 'comment = "keywords that haven't fit
into other groups (yet).";
(viii) a number of language Bundles have this category and many of the changes you'll
want to make will likely belong here;
(ix) now go to the next line (begins w/ 'match') and add a pipe character ('|') after
the last item before the ')\b', then type 'exec', followed by another '|' then type
'compile'--that's it.
Now TM will recognize these two words, but as what? I.e., how does it color them?
In this instance, we told TM they were keywords, so go to TextMate > Preferences > Fonts & Colors, and scroll down until you find the 'Comment' in the Element field. There you'll see it's color, but that's just a function of the Theme you're using at the moment.
In sum, you can change both which words TM recognizes (via the relevant language grammar) and how TM colors that word via the Fonts & Colors tab, which again is Theme-dependent.
And Now For Something Completely Different: if you get tired of purple, check out the alternative GUIs, which (takes about 2 minutes or less to DL and install): http://wiki.macromates.com/Main/AlternativeGUIs. The last one called 'Green Moleskine' by Jason Evers is the only one i've used. Gorgeous as you can see, plus takes like 2 min to DL and install.