It's a strange one, but Photoshop likes to be silly occasionally. I couldn't use the brush size preview all of a sudden on two different Macs, but now it's working again.
All I can suggest is going through all your colour settings in preferences and everywhere else you can think of.
Failing that, You could try a complete preferences reset. You do it by holding these keys down on start-up of PS:
(WIndows PC): Alt+Control+Shift
(Mac) : Option+Command+Shift
After doing that, I would create a custom workspace to avoid having to put all your palettes back the way you want them the next time you need to reset it.