How can I define custom colors for use in ZSH prompt?

I'm having some difficulty configuring my zsh prompt. Specifically I would like the font to have the color defined by the hex code: #87afdf

Currently, I've set up the prompt as follows:

PROMPT='%B[%d] 
➞  %b'

I've attempted to add colors in the following way:

autoload -U colors && colors

PROMPT='%{$fg[#87afdf]%}%B[%d]
➞  %b%{$reset_color%}'

But this only gives me the following gibberish:

$fg[#87afdf][/Users/gregory]
➞  $reset_color

Any ideas on how to proceed would be very much appreciated.