Mars is often referred to as the Red Planet, and it gets its red colour from the iron oxide in its soil. You might choose a red crayon (and possibly orange too) to colour it in...
Mars Colouring Page
For most of us looking up at the sky, the planet Venus looks a yellowish white colour, but if you were close-up it has a reddish-brown surface - so we'll let you decide which colours you choose to colour in this planet!
You might want to find some blue and green crayons to colour in the planet Uranus, which has a blue-green colour due to the cold methane gas present in the atmosphere around it.