The W3C published a full Working Draft for compositing and blending
for SVG and Canvas in August.
MyPaint and OpenRaster compositing is based on the SVG standards, so I've added
all the blend modes the new draft mentions to MyPaint in git so that we have a full set: see the attached image. The new blend modes are Darken, Lighten, Difference, Exclusion, Hue, and Saturation.