Alt. colour based on style

[Morrus]

I'm hoping this isn't a programming question, but I'll start here.

I have two styles; black and white. Default text colours work correctly, as they should (light on the black, dark on the white).

However, I also need to be able to post snippets in news items in a different colour to the rest of the (default) text. I can't select a colour from the colour chooser, as one colour won't be suitable for both styles. So I need a way to select a 'alt default' colour which changes depending on the style in the same way that the default colour does.

So... how would I do that? Thanks!