I just painted mine using Krylon Fusion Black Satin. This paint is specifically formulated to stick to plastic. I removed the valances (see below), cleaned them with the soapy water and then wiped them down with some mineral spirits. I applied 5 coats of paint, waiting about 30 minutes between coats. For the 6th and final coat, I waited about 2 hours before applying.
This is VERY easy to do by removing the front and rear bumpers. I tried to do it without removing the bumpers and the spaces were too tight for my hands. Removing the bumpers sounds like a daunting task, but it is actually one of easiest things I've done to my 4Runner.
Here's a video on front bumper removal:
YouTube
Once you have that off, you can see the clips that hold the valance to the bumper. Use a screw driver to press the tab and slide them out.
I couldn't find a video of rear bumper removal. I did find various pictures and text descriptions of removing it. I took some pictures in hopes in helps the next shadetree mechanic...
In the 1st picture below, you can see a red arrow that points to a 10mm screw you need to remove. The blue arrows point to three push-in clips. To remove those, use a small flat-head screw driver. Insert it into the groove on the top side of the clip and pry the center cap up. Then just pull out the clip.
In the 2nd picture below, the arrows point to 10mm screws you need to remove. The blue screw is special. Once the blue screw is removed, you can move the mud flap out of the way and access another push-in clip shown in the 3rd picture. The blue arrow in the 3rd picture is where the blue screw in the 2nd picture used to be. My finger is pointing at the push-clip.
In the 4th picture, the red arrow points to one more 10mm screw you need to remove. It's over towards the center of the bumper by the hitch and spare tire. IIRC there is also a push clip in this area.
Repeat pictures 1-4 for the other side of the truck.
The 5th picture shows the other screws you need to remove.
To physically remove the bumper, pry the bumper away from the raised retainer clip that is located where the first screw you removed was (the red screw in the 1st photo). Then, give the bumper a tug away from the rear of the vehicle. There are some clips in the segment behind the wheel that will release. The bumper will slide freely and off the back of the truck. Now you have easy access to the valance clips.
