zsh: remove an unnecessarily hard-coded string in colors function
zsh: remove an unnecessarily hard-coded string in colors function

Was playing with recent updates to base16 and noticed this opportunity to
replace a hard-coded string with a variable.

(However, note that due to https://github.com/chriskempson/base16/issues/42 I
can't go through with the upgrade, yet.)

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent