User Interfaces - Colors and Layout

The book Designing Interfaces, by Jenifer Tidwell has a entire chapter on the subject (Chapter 9, excerpts accesible online). The entire book is worth recommending.

One tip to check if your colors have good contrast is taking a snapshot of it and converting to grayscale. If you can't read something, colors were surely bad choosen.

Plus, although it's not about user interfaces, Before & After Magazine can give you some pretty good hints about color, design and related topics. It even has got some free pdf's to download.

Usually, each operating System has user Interface Guidelines. For Windows, have a look here. (Edit: The links in that post are broken. But a Search for "User Interface Guidelines" on MSDN has articles about everything)

Apple has it's own as well. Also, you may want to keep accessibility in mind.

For web UI, I'm going to go out on a limb here and say that the most important color in web design is white, or "light". This is the color on top of which you place dense tracts of content.

Dark text, light background, always, when it comes to your primary content areas.

And the most important rule in layouting is whitespace. Let the content breathe.

Following these two simple rules is worth more than most "user interface usability" guidelines.

And by the way, the MS user interface guidelines are (by and large) horrible. Read Jakob Nielsen, look at Apple design aesthetics, but stay away from the MS "neutral gray/blue crunchbox" 12-step Wizard 10pt text philosophy of UI.

(And I say that as a long-time MS GUI programmer)