Even though the new bumper is factory painted it doesn't necessarily mean that the new bumper and old colour will match 1:1 -> they might have to paint the new bumper regardless which is why it might be preferable to just fix the existing one.
In fact all the manufacturers I've ever worked with send you an unpainted bumper when you order a new one(or any body panel really) and it has to be painted before being fitted, specifically for the reason you mentioned. A good body shop will match the paint that's actually on your vehicle.
In my experience honda parts do match 1:1 and I don't think they'd waste the effort on maintaining a stock of pre-painted parts if the receiving body shop was going to have to paint it anyway.
