but, when i exported it by “Import 3d” tab, the model got this color and i can’t paint it by hand cause the parts are not correctly separated (cause it’s a rigged mesh)
How did you color the mesh inside Blender? Did you use material assigning? By material assigning I mean clicking on some of the faces in edit mode and clicking assign on material tab.
That’s the reason why your colors do not appear when you import them into Roblox Studio. Roblox does not support this. You need a texture image containing the colors.
There are 2 ways that you can fix this.
Baking the textures into an image file. I made a tutorial for this which you can follow: bakingtuturoial on Vimeo
After following those steps upload the texture to the meshes TextureID property in Roblox Studio. But don’t forget to re-export the mesh from Blender to Roblox Studio! Otherwise colors won’t apply properly.