Why not just open your favorite image editor, (seashore is a good free one for this) create an image any size you want, fill it with the color you want, and then set it as your background, just choose "scale to fit" though
you're right. my apologies. when a picture is smaller than your desktop, you can choose the color of your choice to border that picture. i only assumed you could make the entire solid color be the background as a default option.
there is a way to get what you want, though. take a small sample of the color from some source using control-shift-4, and put that snapshot in your pictures folder. select that image to be your background, having it either stretched or tiled.