urethane bumper cracked paint repair

I was rear ended at a stop sign and now have spider web cracking in the paint at the bottom edge of the licence plate opening on the rear bumper of my red 1987 vette.
I have removed the bumper (big job needing long thin arms) and do not see any cracks in the (off white colored) urethane on the inside of the bumper. There is no obvious damage to the bumper or to the frame or rubber egg-crate pad
I would not have fixed it except that it is an innsurance job and I wanted to make sure that the bumper had no hidden damage. I hope that I can get the beltline trim off without breaking it but have been warned that the bolts will probably tear out of the rubber trim. I have applied penetrating oil and hope that will help.
My question is regarding the correct repair proceedure for the paint on the bumper. I have been told there is a special proceedure to relax? the urethane bumper prior to repainting. I would appreciate any information as to the correct way to repair and repaint.
I am suspecting that getting a good color match is going to be difficult. I have noticed that my side mirrors have faded differently than the rest of the body and now have a purplish red color.
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Only think I can think of is using a flex agent in your paint so it tends to bend rather than crack when the bumper flexes.
And paint supply will know what you need.

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