[matplotlib-devel] colors in backend_bases.py

Hi,

I'm working on https://github.com/matplotlib/matplotlib/pull/3393 and 
poking around at _alpha, _forced_alpha, _rgb, and _orig_color members of 
GraphicsContextBase.  I see quite a number of issues:

1)  "_rgb" is (almost) always a 4 tuple which may be better named "_rgba".
2)  "_orig_color" takes on many different value types -- I think it can 
be deleted and the "_rgb" variable used in the one place it is needed.
3)  "get_rgb" documents that it returns a 3 or 4 tuple.  In fact, I 
think it returns a 4-tuple (see point 1).  backend_ps.py is a 
particularly scary example of the 3 tuple interpretation being taken 
with-out further thought.
4)  I suppose there is a good reason for the existence of 
"_forced_alpha" and "_alpha", but I don't understand it.  Since "_rgb" 
is a 4-tuple in common (all?) usage, why can't "set_alpha" merely set 
"_rgb[3]"?

I'm going to be pushing something more to my branch at 
https://github.com/matplotlib/matplotlib/pull/3393 -- deleting 
"_orig_color" and renaming "_rgb" to "_rgba".  But even with these 
changes, the points above give me pause.

(I'm also sitting on irc right now asking these same questions).

Joel