My current theme of choice in Vim is solarized8, which requires true colors support in your terminal. It works perfectly when running Vim inside of iTerm, but it wasn’t working quite as well in Vim inside tmux.

Lots of advice online suggests either making sure you launch tmux using tmux -2, or adding set -g default-terminal "screen-256color" to your ~/.tmux.conf file. Neither of these fixes worked for me, although YMMV.

What did fix things for me was adding terminal overrides to ~/.tmux.conf like so:

set -ga terminal-overrides ",*256col*:Tc"

Huge thanks to rineth for their helpful post.