You could use a gradient:
example:

background: -webkit-linear-gradient(top, #333333, #333333 49%, #666666 50%, #666666);

This is for Safari 5+ and Chrome 10+. For older versions, the code is longer. Just change the "webkit" into "ms" (IE), "mod" (Firefox) or "o" (Opera).

Try this for making de gradient:
colorzilla gradients

Fantastic, thanks guys. I'd had a feeling that it would be a gradient but wasn't sure I could get a perfect line between the two colours instead of a slight blur. I'll give it a go. :D

@cyneWATCH You would use this for best results:

background-image: linear-gradient(#333 0%, #333 50%, #666 50%, #666 100%);