I'll give you an example to get you started. Notice when you view the
HTML on the page (link below) ... the CSS is defined in the <head> section.
You can also make a separate .css file for that, so all of your pages can use
the same style sheet.
You specified background images for those color sections ... they don't need
to be images if it's just solid color (but I'm not sure of your intent, so I left
them as background images).
With my example that has a variety of thing you can play with, you should
Google some CSS tutorials to help get you started.
Here's the example page:
(view HTML source by clicking "View", "Source")