Purple orb

You need to interlock a red light and a blue light together.

 

You mention red, yellow, and blue. Those are the colors of powerups. If that is what you mean than red + blue = Purple. If you actually mean lights, than just do what the guy above me said.

 

Well I'm pretty sure red and blue make purple no matter what it is your doing, that's just how you get it... Red blue and yellow are the primary colors, using them you can get practically any color.

 

but not in halo, green doesn't work

 

This gives a more pinkish color than purple. But it is the closest to purple you can get if you want it to appear as just one light. More information on lights is available HERE

You can Interlock an Active Camouflage with an Oversheild to make a purple powerup. Just be sure to interlock it with a wall or other item so that it cannot be picked up. However you cannot combined a Custom Power Up with a camo to make green. or CPU and OS to make orange