My zones all turn the same colour

[Floydian]

Hi, me again 

I'm having an odd graphical issue with my game. When I lay down zones, they are the colour they should be. However, when I lay down more zones or different colour zones, the colours start going wonky on all the undeveloped zoned tiles. They eventually all change to be the same colour (as if they were all residential or all commercial or all industrial).

I'm running a Dell Latitude D630, Windows XP, with the Intel GM965 chipset.

Will add a picture soon if it is needed.