As long as you are using a Widnows machine, get yourself a copy of Accutrans3D from
https://www.micromouse.ca/
I already had the file save with coloured faces, checking my workflow for the below two methods only took 25-30 minutes total.
Method 1: Converting face colours to UV map
With Accutrans3D
Load model
Align normals on all layers
Depending how many colours you have will be the number of layers, you may need to go through the list of layers one by one to flip normals
Under the tools tab, click 'Convert layer colours to UV and texture' save the image file as a jpg
Go file, save with options
Selesct Save File Type as VRML 2.0 (97)
Check UV, Texture Path and Partial
Click use units, make import unit as millimeters and export units as meters (SW treats native VRML units as meters so the model needs scaling down to 1/1000 of it's original size).
Click save
Find the model and image file
Create an archive zip file containing the image and model file
Upload zip and wait for a bit for confirmation.
Example of the above technique
https://www.shapeways.com/model/335986/13900f0e77821287248c52 e08e29a88b/colour_police_phone_box.html
Method 2: Saving model with face colours.
Again with Accutrans3D
Load model and align normals as above
Save with options as VRML2.0
Check Vertex colours and Vertex Normals, scale as above
Click save
Upload wrl file and wait for confirmations
Example of this technique
https://www.shapeways.com/model/335991/5e65eaedfe3a0d7faf588c 0043ad250d/colour_police_phone_box_vertex_face_colour.html
Paul