I see you have already had some good suggestions and each method has its advantages and drawbacks.
The negative margin approach is a a good method but add some complexity to the site not to mention extra elements. It can be awkward to tweak the design once completed because you have to adjust everything else to suit so maintenance can be awkward. However it does work well when implemented properly.
If you have a fixed width site (or fixed width column) then the "faux column" approach is an easy solution then seldom needs extra markup as it often lends itself to the existing structure. The drawbacks are that you have to make an image each time you want a colour change.
If IE8+ support only is required then you can use the display:table properties for your equal columns and in most cases will need no extra markup.
A technique I often use for full length columns is to use an absolute overlay and was recently discussed in this thread.