Well, I agree with BVC's comments on colors but whether you decide to paint the lower fascia black or body color is kinda irrelavant because adding the part after paint will leave a very unscale seam in a conspicuous area.
I always test to see if these parts can be permanently added prior to body paint by securing them with tape. Then I determine if the body can be added and removed from the chassis assembly without the part moving. If the part interferes preventing the body's removal I try to modify the attachment so I can get it on and off with the body fully assembled. These lower front and rear facia panels are usually molded separate to reduce the need for tool inserts and other complications. Good kit design usually allows them to be added prior to body paint but I always check lest I find myself painted into the proverbial corner.
One additional thing: the chassis and body become stiffer as other components are added to it so don't think bending them appreciably to get them apart is sufficient.