I’m installing my « new » Imac 24″, which I got from my boss who leaved us a week ago. As you may notice, the OsX background can be any picture you want, but when looking in the solid color choice, you only get 10 of then with no dark or black color.
Adding some there is quite easy.

Colors are, in fact, PNG images, 128×128 pixels. They are located in « /Library/Desktop Pictures/Solid Colors« 
Just go there, copy an existing image, change its color with any graphical editor (Gimp ?) and there you go.

OK, this is not a tutorial… but, heh, it’s so damn easy…

OK, OK, I do agree too. Apple should let us chose the color in a color wheel instead of stretching or mapping a small image to the whole screen… Maybe we’ll get that on Snow Leopard soon…