This is a version of https://github.com/altercation/vim-colors-solarized modified to always use true color (24 bit rgb). This means it will work with neovim with the following:

let $NVIM_TUI_ENABLE_TRUE_COLOR=1
set background=light " or dark
colorscheme solarized

Normally this wouldn't work with neovim (as of writing) since gui_running is always false during initialisation.

For more info see the original.