A good text editor you could use is Sublime Text. It's technically free (since the only non-benefit from using the trial version is you get occasionally nagged about buying the full product), and it can do all of the things you'd want from a text editor:
simple, free and fast text editor
Sublime Text can be customized as much as you want using the Plugin Manager available for it, so if you want to keep it as simple as possible you can. It's also kinda-sorta-technically free as mentioned before, and it's very fast.
Using some excellent linting plugins you can install fairly easily using the Plugin Manager, you can have errors highlighted as you type, or whenever you save, among some other options.
colour-coding keywords and variables differently
(I'm assuming you mean syntax highlighting)
Sublime Text can do syntax highlighting fairly well. Here are some examples:
Sublime Text also has build systems and useful keyboard shortcuts and commands which can speed things up quite a lot. Most notably of these features is the command palette, a "center for commands" where you can execute most of the editor's functions by just searching.
Another text editor you could use is TextMate, Sublime Text's "spiritual predecessor." While I don't know the ins-and-outs of TextEdit, since I am a Windows user, it seems like a good Mac-only alternative if you don't want to use Sublime Text.
And if you really want to be able to dedicate yourself to a text editor, you can try Vim and/or Emacs. Be warned though, since these editors aren't very simple to use and require a long-term commitment to use.