If you start vim within a tmux session, you might notice that your terminal’s color scheme “bleeds” into vim’s. This is a result of tmux not using a 256 color terminal. It is covered in the tmux FAQ “How do I use a 256 colour terminal?”. The following is a complete excerpt but edited for formatting purposes.

If you attach to your tmux session and echo $TERM says something like screen, then you know something’s wrong. Before attaching to your tmux session, do echo $TERM (it may be something like xterm-256color). This is what you want to put for default-terminal.

Provided the underlying terminal supports 256 colours, it is usually sufficient to add the following to ~/.tmux.conf:

set -g default-terminal "screen-256color"

Note that some platforms do not support “screen-256color” (infocmp screen-256color will return an error) - in this case see the next entry in this FAQ.

tmux attempts to detect a 256 colour terminal both by looking at the colors terminfo entry and by looking for the string “256col” in the TERM environment variable.

If both these methods fail, the -2 flag may be passed to tmux when attaching to a session to indicate the terminal supports 256 colours.

tmux -2 attach