I'm currently working on an image slider where each slide needs to have its own background color.
Letting editors choose a color is easy with the Color Field Module. Just install it, throw the Spectrum Color Picker into your libraries folder and add and configure a Color Field where ever it's needed. You can then simply access the user-entered color as a hex string in your template and use it to your liking (yea, that's one of the rare times where it's ok to use inline style, at least in my book).
Convert Hexadecimal Color Values to RGBa
I needed my color to have alpha transparency, however, the color value is only available as a hexadecimal string. Luckily I found this handy snippet to convert hex values to RGB(a), so thanks a lot to mekshq for sharing.
Just put it into your
template.php and you're good to to to use
hex2rgba() anywhere in your theme.