bug#13686: hi-yellow vs. hi-lock-1

[Drew Adams]

> In lighter vein, I still remember the old days when I looked at
> "hi-yellow" and thought that it was a "very bright yellow" 
> and searched for "lo-yellow".
> 
> No, I am not making the above story up.

;-)

Not very important, but is there a reason why the prefix `hi-lock-' is not used
here as elsewhere?  (And perhaps it should be added to the defgroup.)