ggordan/GutterColor — pull request #18 by bwinton (2014-03-13 22:07:33+00:00)
Title: Handle rgba, hsl, and hsla colours, too, by mostly passing them to ImageMagick.

This seems to fix issue #15, at least for me in ST3.
(ImageMagick handles all sorts of colour formats, as shown in http://www.imagemagick.org/script/color.php :)