Hi, here is what I understand from the error ( cause I don't use blender at all )
Probably there is a material on you model that has a sub material ( such as diffuse, specular, normal etc) that is not an SC2Material type but a SC2Color type. Each material and submaterial can be named.
One of them is called "col" and can only be exported when it is named "color".
Another thing is that it is bad practice to merge the meshes together as it screws up the vertex weights for animation. I may also be that certain submeshes required alpha channel and others don't. This actually speeds up the drawing cycles of the engine.
Having multiple materials is not slowing down as the number of textures are not loaded into memory multiple times.
Layers which I have in use seem to be Diffuse, Specular, Emissive, Normal. also there appears to be a Gloss active. I must probably me stupid, but I don't understand what it means by "has color named 'col'" and after that "must be named 'color' or 'alpha'" I have never explicitly made any change in any such material.
But I've been able to make an export now, by eliminating the mesh in question. Now to go on further investigation.