For bare plastics, it's best to use an adhesion promoter or plastic primer first or you might risk the new paint flaking off. Use a high build primer over body filler.

For the bumper, if you're doing a proper job yourself, sand the scratch p180 grit sand paper until the scratch is completely gone and you can no longer feel it, then sand over the areas where the scratch was with p320 grit then p400 grit sand paper (you can use wet sand paper for the 400) to remove the scratches left from the 180 grit and feather edge the surrounding area, making sure you keep your hand flat or use a sanding block.

Use a gray scotch brite with either scuffing paste or just water and scuff the whole entire bumper with it until the shines gone, making sure you get in to all the corners and hard to get areas with the scotch brite. Wipe over the area you sanded where the scratch was with a wax and grease remover, wait for it to dry, mask up around the area the scratch was, spray 2-3 even coats with high build primer, leave it over night to dry. Mist some guide coat or satin black paint over the primer, then using a block or flat hand wet sand the primer with 800 grit wet sand paper until all the satin black/guide coat is completely gone (if there is any guide coat left, it'll be an imperfection that needs sorting out or it'll possible show through the paint in the end), make sure you sand past the edge of the primer so that it's feather edged in to the existing paint.

Wipe the bumper with wax and grease remover and it's now ready to be painted, blending the new paint in to the existing paint and then clear coat the whole entire bumper. For the primer color I'm not entirely sure, I usually just use my light primers with my light colors and dark primers for dark colors. There is a few different ways you could tackle the bumper, if the scratch isn't too deep you could use a 2k stopper over the scratch and wet sand with 800 grit if you want to skip priming it.