Hi!
To use texture in a material, all the objects with this material should have UV mapping.
In SketchUp it is impossible to create and export UV mapping for objects with an untextured material. So, you have to set some texture already in SketchUp. Then, in Shapespark you will be able to change the texture to some other texture. If you want the material to be initially solid color, use a solid color 1x1 texture.