I absolutely love this idea and am surprised no one has come up with it before. The Squidarella umbrella by SquidLondon turns your boring black and white umbrella to color when it comes in contact with water. I would totally get one if I could justify a $38 umbrella on URBANtrim. You can also get it at SquidLondon's shop.
You should also check out the SquidLondon color changing rainwear: