Just my experiences...
I've had to do 'bumper repairs' on the wife's car more than once (people like to hit her in the parking lots) on her 2000 Subaru.
After sanding down all the edges and surfaces, I use factory touch-up paint (yea, those little bottles with a brush) and reduce (cut) it to about 50/50 and spray it with an airbrush.
In my case, the paint is a 'metallic' and the new paint is always a tad darker than what is on the bumper. So, instead of spraying a clear coat, I let it sit for a couple of months to fade then clear it. Which reminds me, I gotta shoot this years 'fix' this weekend.
Is it perfect...no. But it sure looks a LOT better than all that black ABS plastic showing thru.
Bill