High temp light diffusing paint?
 

Frost it and you will retain heat and possibly do damage, talk to a
manufacturer of fixtures or bulbs.

High temp light diffusing paint?
 

Add a piece of frosted glass.
Frosting your glass can weaken it and MH fixtures need a sheild in case
of "non-passive failure."

There are some theatrical materials that come in a wide range of
frostings and colors. If the heat isn't too great they would be ideal.