Suggestions: Customizable header image and style colors that are different for each page

I think your options are to actually use custom fields, but you can make them user friendly.

The header is relatively simple, you can even have a preview or a selection of images to choose from, have a look at how the default twentyten theme enables header options, very user friendly, no urls are used , simple upload or choose an image.

As for colors, hex codes are a pain , but it would not be hard at all to add something like a jquery color picker ( like http://gusc.lv/jquery/gccolor.html) to the fields you define.