I printed cellphone cases for myself and my wife, but since the cases and phones were both identical we sometimes got confused momentarily (until we looked at the home screen, of course!).
So I decided I should try to dye my case. After a brief Q-and-A on the Rit Dye forum, I decided to try Rit DyeMore - which is specifically formulated to dye synthetic materials (i.e. plastic, not cotton or wool).
The 7 ounce bottle of dye is sufficient to color 2 lb of fabric, and my cellphone case only weighs one ounce, so I did some calculations and came up with this brew ):
1.3 tsp (6.4cc) of dye
3 cups (0.7 liter) of water
The dye needs to be heated to near boiling to work. I used a disposable aluminum "loaf" pan to hold the dye and a disposable aluminum "turkey" pan as a shield to catch any spills or splashes. (The amber color in the lower right corner of the turkey pan is a reflection, not anything actually present in the pan).
The dye also need time and agitation to work. I gave it 1/2 hour in the simmering solution, moving the case back and forth gently the whole time with a piece of wire shaped into an appropriate hook (partly visible in the above picture).
The result was great!
By the way, the Ninja Flex Semiflex was printed through an E3D V5 nozzle at 250C, with a 0.4mm nozzle and layer height of 0.2mm. I sliced it in Slic3r, using a maximum volumetric speed of 2.5 mm^3 per second.