I tried Mothers Back to black, and found it to do very little more than armorall. I would recommend Forever Black....it is a black dye in a bottle with a sponge top applicator. It will turn grey trim to black with a couple/few coats. Very easy to apply, no need for tape or sanding. I bought it at IPD. Made my grey trim on my 240 look really great, and it is holding up fine after a year.
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92 245, 1965 122 Wagon, past:95 850 Turbo, 92 245, 88 245, 82 245 Turbo, 71 164e

3 things to try:
1. Easiest/cheapest: "Mother's" brand Back-to-Black. Short term, mixed results, but if it's good enough it's good enough and no more trouble than the regular wash-n-wax of the car.
2. Dupli-Color trim paint. Cheap rattle-can stuff but can look okay with decent prep. Made to work on soft bumper covers.
3. Actual paint job with a flex paint. I'd suggest removing all black trim from the car and doing it in one batch; that way you also don't have to worry about masking so much. Drop the lot off with the local paint shop, pick up and install.

If the bumper covers are the kind that match the paint of the body, you are pretty well stuck with option #3 or a similar process. But if not, go to your local auto superstore and try option 1 or 2 first.

See the FAQ: you can lightly sand them for adhesion and spray an auto bumper paint on. Bumper paints have flex additives to prevent cracking.
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See the 700/900 FAQ at the drop-down menu above right.