They are dyed. You can dye them using RIT dye. I really need to do a how to video on it, but you can find the process on youtube. For small items like knobs, it is easy, but you need a much bigger pan for the end handles and lid handles.
The good thing about dye is that it soaks into the plastic and if the knob gets scuffed or nicked, it won't expose the gray underneath.