Huh, that is quite odd. Have you set tmux to true colour? If not that may be the issue, though I don't see why it would select those specific colours.
Incase you haven't, here's the configuration to do so. Place the following in your tmux.conf located at ~/.config/tmux/tmux.conf
or ~/tmux.conf
set -g default-terminal "screen-256color"
you should have to run tmux source ~/.config/tmux/tmux.conf
to get it to reload
You may also have to add the following to your nvim config:
set termguicolors
or set TERM
to xterm-256color
in your shell (example: export TERM="xterm-256color"
for bash)
Though I doubt either are your problem.
Edit: fixed neovim configuration