> Let's choose second option. Don't allow assigning "the other color" seems like a better solution to me. Furthermore, su-v had an idea to make mouse cursor follow current mode change (or in the case of using > Dropper tool directly - modifier key change).
>
> I had 10 minutes of spare time this morning and... may I present you PIXMAPS! Ta-da! :)

I agree - don't allow assigning "the other color" seems more natural.
Thanks ! The pixmaps really make it easy to tell what is actually being set.
v2 patch for above (with pixmaps) attached to bug report.