E421: Getting color name not recognized in perfectly valid statement

I want to change the colors of my parentheses colorized with Better Rainbow Parentheses. The colors are specified like this: ``` let g:rbpt_colorpairs = [ \ ['brown', 'RoyalBlue3'], \ ['Darkblue', 'SeaGreen3'], \ ['darkgray', 'DarkOrchid3'], \ ['darkgreen', 'firebrick3'], \ ['darkcyan', 'RoyalBlue3'], \ ['darkred', 'SeaGreen3'], \ ['darkmagenta', 'DarkOrchid3'], … \ ] ``` The function changing the colors in the plugin is this: ``` func! rainbow_parentheses#toggle() if !exists('s:active') cal rainbow_parentheses#load(0) endif let afunc = exists('s:active') && s:active ? 'clear' : 'activate' cal call('rainbow_parentheses#'.afunc, []) endfunc ``` Since I’m using Solarized I want specific hex colors, so I got this: ``` let g:rbpt_colorpairs = [ \ ['yellow', '#b58900'], \ ['orange', '#cb4b16'], \ ['red', '#dc322f'], \ ['magenta', '#d33682'], \ ['violet', '#6c71c4'], \ ['blue', '#268bd2'], \ ['cyan', '#2aa198'], … \ ] ``` Yet, on restart and turning the plugin on, I get this error multiple times: ``` E421: Color name or number not recognized: cterm=orange guifg=#cb4b16 ``` Even better, doing the :exe done in the function manually works like a charm, like: ``` :exe 'hi default level4c ctermfg=orange guifg=#cb4b16' ``` Even with concatenation it works: ``` :let foo = '#cb4b16 :exe 'hi default level4c ctermfg=orange guifg='.foo ``` What could be wrong?!