It's a term that is used to refer to people who are often excluded from media in Tumblr terms:
Why are black, asian, latino, and native americans not in this piece of work?
Why are there no PoC in this particular work?
And PoC have their own ways of being discriminated against (yes even among each other).
Sounds better than "Why are there no colored people in this piece of work"
It's not exclusively Tumblr but people use PoC over "colored" the most on Tumblr the most.
If you want to used "colored" go ahead but that doesn't mean because you find it okay other people do.