Tell me about it ... i've also encountered this issue with my
latest experiments of
full color sandstone models ..
I also found out that wrl is parsed differently by many popular and commercial 3D software - cant tell why.
The workaround i finally found is by selecting each texture/material in my case in Sketchup , under the paint bucket menu --> In model .
Then right-click on a texture and click on select - this will show you on the model what parts contain that specific texture.
Export that selection as part1.obj then do the same for the next texture as part2.obj etc...
Now load each OBJ part using Meshlab , one after another (open as a new layer) then flatten all layers and save as one .wrl file
That did the trick for me - as you can see on the links above all textures are in place.
If you can't do the same with your 3D software, the easy way is to export your model to OBJ file or COLLADA , import it to Sketchup then follow this workaround.
If you have Sketchup Pro the file formats you can also import/export are .wrl, 3ds etc..
BTW, obj files seems to work good all the time when loading them to different 3D software , perhaps shapeways should enable OBJ uploads for color models - maby this could help avoid this "channels" headache.
Hope this helps.