Clogged sink drain after washing paint brush and roller
I would suspect that it's something other than paint. We use our kitchen sink to wash out brushes and have never had issues with it clogging. You would have had to have washed a lot of very dried out paint down in order for it to have clogged.
If plunging hasn't solved the issue, you could try any of the chemical drain cleaners, or you could try a snake or auger. Hand operated snakes are fairly inexpensive and are handy to have around in general and don't have any of the drawbacks/concerns that people have with chemicals.