I'm currently using the <extra> element of collada, inside a <profile_COMMON>. It would be cool if you could add to 2.80 that it just reads that and remembers it (even though blender can't use it) when you import something from collada - so it can export it again for the profile_COMMON in an <effect> with the same id?
Apr 11 2019
Hi Gaia. I'm not even sure if collada supports having a diffuse color and a diffuse texture at the same time? At least, I couldn't find it. Other formats do.
At any rate, I'm only testing with textures; this is for Second Life, and SL almost always has a texture on any surface, but also allows a color at the same time.
Just tested 2.8 - that's three steps backwards. That version doesn't even export anything regarding materials (like alpha masking, colors, emission, etc).
Feb 16 2015
A few months have passed. Anything yet?