Well, it's not the best solution, and it's not strictly using CSS, but you could make a small image that contains the transition from 1 color to another, and then stretch it to the desired size ...

ColorHexa is a color encyclopedia, with CSS gradients, blends, schemes and more February 3, 2012 - 11:06 pm Color, one of the seven elements of art, is one of the most important aspects of any design.