Problem

I like making diagrams where my patch edge is solid but the fill is semi-transparent - in my case usually to fade out the color, but it's also useful to emphasize boundaries

Since alpha applies to the whole patch, the way I do this now is by converting my HTML colors into RGBs and appending an A

edgecolor=x_color, facecolor=(*mcolors.to_rgb(x_color), .10)

ETA: apparently I could also just do facecolor = mcolors.to_rgba(x_color, .10)

Proposed solution

Allow alpha to take a (facecolor, fillcolor) tuple for patch methods. As an example of prior art, we allow vectorized/separable alphas in imshow as of #14889