Worst time of the year to do paintball is in the heat. Your mask gets proper steamed up
Diving tip: smear a very thin layer of soap (fairy liquid/shampoo) on the inside of the lenses will stopp 'em fogging, but it has to be a very thin layer or it'll inhibit your vision.
Yeah they get you to do that after every game when doing it in the summer which is a pain but its more the fact that the adrenalin and activity gets you sweating your rocks off which is very uncomfortable when you are strictly prohibited from taking your mask off.