Can you mix red and green antifreeze?

Not as simple as "No.".
It doesn't matter what color it is. It matters what base chemical it is. Ethylene Glycol or Propylene Glycol are your 2 choices. Use one or the other. If you have mixed the 2 different base chemical types, you will have to flush the entire system. What you will expect to see from mixing the 2 is a sludge build up on top of your water in the radiator. This is usually black, or dark gray. Gray usually only because after the separation process in the lubricants found in coolants, the 'oily" sludge will draw moisture causing the grayness.
Also, try to use PG (most of the time it's red) in engines/cooling systems that predominantly incorporate the use of aluminum parts. This will help preserve the seals and help against corrosion caused by electrolysis.