Repainting body parts
Do it yourself! Touch-up brushes are unusable but Holts (England), Multona (Italy) or Dupli-Color (Germany) sell the same acrylic paint in 400 ml spray cans, and those are excellent. You can find the matching paint in their catalog based on your car's paint reference number. You may have to check more than one of those, their color range is not exactly the same (for instance, I could find the exact match for my Red of the Long Valley only in Dupli-Color and the Anthracite Grey used on the upper frame of the doors only from Multona). These lacquers are enough for small scratches but to repair larger areas, especially if the plain metal can already be seen, you will need the primer of the same paint family, too. The primer is a thicker paint, acting as a filler as well; and being thicker, it is much less transparent than the lacquer, one layer of the primer covers the surface much better than two or three layers of the color lacquer. Thus, if you do use the primer to make the imperfections and color differences of the underlying surface disappear, you will be able to spare a few layers of the lacquer and to obtain a better finish.
These primers generally come in white, grey and brown, select white primer for white cars, grey for light and brown for the dark colored ones; otherwise you might need one or two additional layers of the lacquer to match perfectly. Metallic finishes also require a clear lacquer coating on the top.
Note that the original bumpers are different both in color and texture, the paint being in the plastic material itself and not a coating on the surface. Replacement bumpers, however, only come in grey and have to be painted to a matching color. The acrylic paints mentioned do not adhere to the polypropylene bumpers: you can try it on the inner, invisible side of the bumper: spray some paint onto it. Stick a piece of adhesive tape on the surface next day and see whether the paint peels off when you remove the tape. If it does (which will probably be the case, unless your replacement bumper is made of something different than polypropylene), you have to coat the surface with a plastic primer first. I used Holts Spraymatch Plastic Primer (it was excellent but it is transparent so it's not always easy to see which areas are already covered) but Sikkens and other professional paint manufacturers also have their own version. To be safe, repeat the adhesive tape test both with the plastic primer and then with the primer plus the acrylic color paint.
There are special bumper paints as well (Plasticote is one of the companies producing such paints) but you might not find the exact match for the color of the car. If you need white, grey, black or are satisfied with a close but not exact color match (if you paint all bumpers, this is not necessarily ugly), they might be useful.
Read and follow the instructions very carefully; the spraying distance is of utmost importance (if you go closer, the paint will not cover the surface evenly and may become too thick and run, if you are too far away, the paint will haze), and keep the 'several thin layers instead of a thick one' principle. I know from my own experience that, when you spray and the finish isn't yet perfect, it is very tempting to add a little bit more paint right then. But resist the temptation, never go over the same area twice in a row. If it doesn't cover correctly, then be it, you'll add another layer later. It can be repainted after only 10-15 minutes, so there's no need to solve everything with one layer. Rub the surface down with a rubbing compound (eg. Turtle Wax Rubbing Compound) or very fine wet abrasive paper before the last layer(s). Try out the whole process on the inner surface or bottom side of the body panel or bumper beforehand.