There are two different ways to provide color data for a mesh asset -- Textures or Vertex Colors. Read on to learn their differences and how to properly upload them. Note, If you have a choice between providing textures or vertex colors, textures are the better choice, because they provide a greater level of detail.
If you've already uploaded a textured mesh and the textures aren't being displayed, please see the article How to identify and fix a missing or unassigned texture file.
If you have a mesh with texture files (typically .jpg or .png files), choose the "Photogrammetry" type during upload. Note, choosing "Photogrammetry" will cause any vertex color data in your mesh files to be ignored, but this is not a problem with textured assets, because the color data will be provided by the image files that you upload.
If your mesh files contain vertex color data that you'd like to use rather than textures, choose the "Sculpt" type during upload. As mentioned above, uploading a textured mesh is preferable, if possible.