You will need to compile tmux from source, you need tmux v2.2 at least.
Also need to compile Vim from source, see https://github.com/Valloric/YouCompleteMe/wiki/Building-Vim-from-source
Then you need to get tmux working with TrueColor. See https://deductivelabs.com/en/using-true-color-vim-tmux/, especially the comments about .tmux.conf. And https://github.com/tmux/tmux/wiki/FAQ