Grey Model in Multicolor preview

Hi, i am trying to print a 3d model in multicolor. i have try to upload in .x3d, .ply, .dae, . stl, .obj+.mtl but my model always appear in grey. but the colours appears in blender.
(created in nomadsculpt on ipad then imported in blender in .gltf2 format then exported in all the mentionned formats)

i am not sure what am i doing wrong?
Many thanks