Add color use cases for valid and invalid fields. 

This change reverses the generic `error` use cases from red text on a white background to white text on a red background.

I'm concerned about this because I can foresee somewhere using just the `error` use-case for text, but on a pink background - if this change is merged, then that would become white text on a pink background.

I think as a general principle, the uber-generic use-cases for text should be dark enough colours that they would be legible without also specifying their background-color, on a white or pink background.

@danskinner, @pauloneillft, what do you think?

Thanks