For ages I've used Apple's own Terminal.app, but this is pretty janky and I don't know that I care to fix it. I'll just try some other terminals and see what the state of the world is.
Left: NeoVim in tmux (screen-256colors) in Terminal.app, the colors look fine.
Right: NeoVim in Terminal.app (xterm-256colors, without tmux), there are far fewer colors and some of it is so low-contrast it's unreadable.