PVC contains plastifiers, this is the major problem maker for durably painting it. I general a painter will say for paiting PVC that is situated outdoors, which is the most common question, to wait at least 1 year if it is new PVC. Than after cleaning you still need a primer for plastic first than paint it. With the correct primer the finish will never realy be the problem.
In your case i wouldn't go through the hassle painting it. Google for black Heat Shrink Tube in the correct diameter. Shrink that over the tube and cut/dril open the holes again. It will be permanently black after that and willl never damage.
Than if you still have a piece of grey end cap showing, use a black permanent marker for that.
If you insist on painting, i guess a black bumber spray from the automotive industry is a good option. Most modern car bumpers are PVC. Once the paint is fully cured it most likely will be inert.