I've met many people from all ethnic backgrounds - but never one who described themselves as a person of colour. Sounds like a piece of posey pseudosocial **** to me, I'm afraid. As far as I'm concerned 100% of all prisoners who were ever locked up all over the world were persons of colour. If you really mean people outwith the ruling caste, why not say so?
"People outwith the ruling caste" is accurate but really long.
POC might be something that Americans use (I live in America) but I've heard American "non-whites" refer themselves as POC. So it may be limited to their experience but I've always used it out of respect for them, since they are the people I most interact with. I'm sure if someone from another area asked me to use something else, I'd use it.