The problem is that in your mesh, some of the triangles do not have textures. Likely this is inherited from the scanning software you are using. These get a default color, which yes is probably white or red in most software.
Meshmixer can repair this but you have to do it manually. An easy way is to paint the grey triangles with Select, then expand once by hitting ">", then do a fill by hitting "f".
This will replace the area with new triangles that have colors interpolated from the surrounding area.
(This is the most precise way, but you can also use a bigger brush, if the precise shape isn't so important and you want to get done quicker).
Will be fixed in 3.1, for now use email@example.com
For other issues please send your model, screenshots and steps to reproduce.