There are a lot of other things to consider when choosing a color scheme. Accessibility for instance, colorblind people will not be able to see certain colors that stand out perfectly fine to the rest of us; typically they'll have to override your color choices in that case but if you do it inside an image that's not possible. Readability is an issue; if someone is going to be expected to spend a lot of time reading text on your site you want to try your best to not cause undue eyestrain, which is one reason to stay away from dark backgrounds, especially against very highly saturated colors, even if they look nice stylistically.
