Painter's Servant

As Painter's Servant enters the battlefield, choose a color.
All cards that aren't on the battlefield, spells, and permanents are the chosen color in addition to their other colors.

Niv-Mizzet Reborn

Flying
When Niv-Mizzet Reborn enters the battlefield, reveal the top ten cards of your library. For each color pair, choose a card that's exactly those colors from among them. Put the chosen cards into your hand and the rest on the bottom of your library in a random order.