*Warning* Polishing bakelite is highly addictive. Do not continue unless you want to spend hours in antique shops scouring for yellowed bangles just to polish off the patina and see what color is underneath. You also may annoy your friends by constantly showing them before and after pictures. They will not fully understand.
Ok. Think of polishing bakelite like polishing silver. Over time a patina covers the surface, changing the outer color. Most dealers don't polish bakelite before selling, so you'll often see colors like yellow, green, blue, red, and black. Generally dark, with the real color hiding underneath. It's not always the case, reds are usually just red, but often, if you look on the inner rim of the bangle, you can see a hint of the original color (see below photo).