synicons.vim adds highlighting for vim-devicons in NERDTree.
Add this to your .vimrc
Plug `her/synicons.vim`
Install with :PlugInstall
You can change the default colors by setting the highlight groups in your vimrc or colorscheme.
:hi DevIconRed ctermfg=1 guifg=#800000
:hi DevIconGreen ctermfg=2 guifg=#008000
:hi DevIconYellow ctermfg=3 guifg=#808000
:hi DevIconBlue ctermfg=4 guifg=#000080
:hi DevIconMagenta ctermfg=5 guifg=#800080
:hi DevIconCyan ctermfg=6 guifg=#008080
:hi DevIconWhite ctermfg=7 guifg=#FFFFFF