Screwed up way of working in photoshop.

[The Lost Global Mod]

you can't really set it in the settings. The only way i can think of is that your "alt"-key is stuck (thus always setting the color to the second one), so try to press it a couple of times or try a different keyboard.
Otherwise you can check your keyboard settings of your OS, maybe something is messed up there (if possible set them to default then).

Last bet is to reinstall photoshop, but that's the last resort (by Papa roach).


edit: oh god ninja'd. Well i use photoshop cs5 too but never had the issue.