More specifically, most devices will want _4:2:0_ Y'CbCr. It's highly unlikely that FFmpeg gave you anything that's not Y'CbCr (H.264 can technically encode RGB, but I've never ever seen it in use). And if it chooses 4:2:2 or 4:4:4, you get a warning.

FWIW, if you really want to encode your video in Rec. 2020, you'd almost certainly want a 10-bit encode, and then you're outside what e.g. iOS devices will render again. Rec. 709 is the safe choice.