An idea from a deck I threw together in 5 minutes. It still can be improved...
Creature - Wizard
Tap: Target creature becomes the color of your choice until end of turn.
I found it interesting that with gating being so color dependent, no one is playing color manipulation. This card can screw up gating. An opponent has a cheap red creature out on turn one, hoping to gate it on turn 2. As the card comes into play, change the color of the other card.
You: Island, Tidal Visionary
Them: Mountain, Raging Goblin
You: Island, Whatever
Them: Forest, Horned Kavu
You: "as Horned Kavu comes into play, Goblin is a blue creature."
End result: Horned Kavu goes back to the hand, and the mana is wasted.
This card also combos well with others. Phyrexian Bloodstock is a 3/3 for 4B that destroys a white creature as it leaves play. Using Tidal Visionary, you can make all your opponent's creatures white. You just need to get the Bloodstock out of play. I use Waterfront Bouncers, Unsummons, Boomerangs, Repulses, and Nightscape Apprentices remove it.
And, with Llanowar Knight running amuck, no problem. Just make the Bloodstock any color but black.