issues when coloring only descriptions in menu list

[apfelsinenhain@xxxxxx]

Hello.

I wanted to have a different colour for the per-match descriptions in 
the menu list, but I can't get it to work in all cases.

- start a clean shell with `zsh -f`

- execute:
 > zmodload zsh/complist
 > autoload -U compinit && compinit
 >
 > zstyle ':completion:*' list-separator "XX"
 > zstyle ':completion:*:default' list-colors '=XX*=31'
 >
 > _a () {
 >     _arguments \
 >       '(--first -f)'{--first,-f}'[first should be red]' \
 >       '--second[second should be red]'
 > }
 > compdef _a a

- try:
  - `a -<tab>`  WILL show color
  - `a --<tab>` will NOT show color

This doesn't just happen when using `_arguments`. Using `_describe` and 
those specific zstyle's also doesn't work.

Screenshots + more tests & examples can be found here:
https://gist.github.com/Orangenhain/c9ac6ea47f187589813b9887c300a897

Is that a bug? Am I using list-colors wrong?

Thanks.