NeoVim + Tmux with true colors on iTerm2.

Use Pathogen, Vundle, Neobundle or your favourite Vim package manager.

To install with Vundle, put

Plugin 'sonph/onehalf', {'rtp': 'vim/'}

in your .vimrc, restart vim then execute :PluginInstall. This will install both the color schemes and vim-airline themes.

Download the files in vim/ and put them in their respective folders (./vim/colors/ and ./vim/autoload/airline/themes/)

Put colorscheme <scheme> and let g:airline_theme='<theme>', if using airline or let g:lightline.colorscheme='<theme>', if using lightline, in your .vimrc to set the color scheme and airline (or lightline) theme. Make sure you have syntax highlighting on, and 256 colors set. Vim version >= 7.4 recommended.

For example:

syntax on
set t_Co=256
set cursorline
colorscheme onehalflight
let g:airline_theme='onehalfdark'
" lightline
" let g:lightline.colorscheme='onehalfdark'

By default vim only allows specifying one of the 256 (8 bit) predefined colors (wikipedia).

If you want to match colors in vim and in your terminal exactly, you must enable true colors (24 bit).

In vim/neovim, use set termguicolors option:

if exists('+termguicolors')
  let &t_8f = "\<Esc>[38;2;%lu;%lu;%lum"
  let &t_8b = "\<Esc>[48;2;%lu;%lu;%lum"
  set termguicolors
endif

If you use tmux, you must use version 2.2 or newer. Put this in your config:

set -g default-terminal "tmux-256color"
set -ga terminal-overrides ",*256col*:Tc"

(source)

To test if your neovim/tmux/terminal combination supports true colors or not, use this test script: