When redeeming the Channel Reward: Set the Mood you can type in a specific colour and it will set it as the background for Shambo's green screen in the camera box on stream for 5 minutes. Hex codes are also supported, however if you're not familiar with those, you can alternatively use any of the standard HTML colours listed below. Type or copy and paste the name into the box when redeeming the reward to select that colour
If you can't decide you can also use Random which will pick a random colour for you. You can also use Super Unicorn (or sprunc for short if you prefer) to set the background to a rainbow!