I've not worked with bakelite in pens much, but some have some family members who are very familiar with plastics. Combine that with some internet research and comes up with this:
1) Bakelite (and some other early plastics... heck even a few modern ones) are Phenolics.
2) Phonolics will form a layer of phenol alcohol on the surface when exposed to UV light.
3) Phenol alcohol is a yellowish-brown color and thus will add a yellow tint to the underlying color.
4) Phenol alcohol actually blocks UV light so the layer that is yellow is very thin, therefore in our blanks it can be turned off.
So here is the kick - we either need to find a way to keep the UV off the pen or understand that the "patina" on it is likely to return. How much it returns and how quickly depends mostly on the amount of UV exposure.
