Unlike .dae, .stl does not support textures. It carries only triangulated geometry. Also the texture is irrelevant to 3D printing. You choose a material from the printer.
Interesting. i.Materialise allows you to choose the color from a list. It doesn’t require any sort of texture file. Are you sure Shapeways requires it?
This is what I see after uploading the stl of my version of your model.
They accepted the stl file as is, went through picking matl, color etc. and placed an order for it, didn’t realize that Materialise was a 3D printing company, missed that in your original post, will give them a try next time (they’re less expensive 
). Thanks again for all your help.