Huh. I never have any problems with watercolors! Usally a very simple level or curves adjustment would do.
I have a crap scanner. Not even a standalone one, it's one of those 3-in-1 printers that happen to have a scanner attached to it. Also it's around 10+ years old.
So when I scan anything, it looks like this:
That is pretty horrible.
So CTRL+L to bring up the level adjustments from this:
And that is usually all I have to do. This is fairly true to color.
If you want you can also color balance it with CTRL+B.
Here is an example of what color balance CAN do if you want. This is NOT AT ALL true to color but CB is useful if your want to fix colors post-production:
I don't know what is your Photoshop level but if you want to know more about lvl adjustment, this is a good read: http://www.cambridgeincolour.com/tutorials/levels.htm
Hope that helped? :D