[0xb100db1ade]

I don't have zsh setup on the OS I'm running currently, but a simple example is that the light-grey autocomplete text from zsh/oh-my-zsh shows up on i3-tmux but not tmux (at least at the time of writing).

I honestly don't remember the exact details, and maybe it's now fixed.

[abcdef123xyz]

Most likely you are using the wrong TERM so tmux doesn't have the correct information about what colours your terminal supports.