This would be intended behavior. If you look at the model under “Solid” viewport shading you will see the models’ part color as it was in Roblox, but under “Material Preview” or “Rendered” shading you’re looking at the .png equivalent of the diffuse texture Roblox uses for those materials, which is uncolored, and won’t have any alpha channel without modifying it so it will stay the white as it’d be if you looked at that textures’ original file.
You could edit the texture to be the color you want or play around with using Mix nodes under the shading tab to mix the texture with the solid color, one of them would need an alpha channel I believe though.