One of the most common things we do as art directors, design directors and creative directors is to comment on PDF's.
In doing that we have to make boxes or circles to show where we want to make changes.
Almost every PDF editor under the sun has Red Stroke, NO Fill as default for this reason.
In Acrobat I have to click 3 times before I can get to set any object as no fill.
I'm really happy to be able to change fill colour of my rectangle or circle, but at least have one of the colours in the default colour picker be transparent instead of having to double click into the Mac OS colour picker.
And in fact why am I having to mix the colours in the Mac OS colour picker anyway.
What is this like 1997?
Considering every other adobe program has an extensive Adobe colour mixed and colour selection tool built in I find it very strange that you are relying on the MacOS colour picker to do this in Acrobat.
(And please don't tell me to turn down the opacity because that affects the stroke aswell. I just want a regular old, red line rectangle with no fill.)