How can I paint over a bitumen floor?
It seems similar enough to "blacktop" (asphalt, tarmacadam, etc...) to apply the advice given in this greenhouse manual (page 7) - basically to dust with portland cement (get white rather than gray, if you can find it) and then sprinkle/mist with water. Certainly the product linked appears to be congruent with common driveway blacktop sealer. I have only read the advice in question, not tried it. 

Tennis court toppings should also work, I believe, and are commonly available in green, blue, or red - and possibly many other colors.