Adding colors to the chats makes the experience more personal, as well as bringing customization to the chats. It is something present in Signal, for example:
The idea is to individually define that the chat bubbles of user “A, B, C, D…” in his conversation to be blue, purple or any other predefined color. It is a design feature that brings a beautiful visual aspect.
we had these colored bubbles, however, we decided against them at some point, for several reasons. iirc ux-wise, there were some problems, eg. when two contact get the same color (you can change it - but you first have to know that).
having colored bubbles also raise the implementation effort - you have different contrasts, have to take care that things are readable etc.
finally, we support background images - and this does not fit well with colored bubbles.
but - the contact-colors are not completely gone 
you have them in the avatars and also the name of the contacts are drawn differently (you will see that in groups only)
but all in all, we decided for a more decent look.
eg. when two contact get the same color (you can change it - but you first have to know that).
What if the colour is made at personal side (not transmited to the other users which is what you seem to suggest with it, that other person choose one and is showed to you because it is transmited with the next message, etc) and a differencial status is always maintained or mandatory between bubbles for accesibility?
This last thing of accesibility is what Pleroma choosing themes, and I think some VSCode plugins, already have implemented for color selection.
In Pleroma-FE this is showed as a difference in “contrast ratio”. (Check warning and other icons below each one of the selected colours in which extra info is showed by putting the mouse pointer over it an instant)
