Lost in UV, painting, import, export

[spacepainter]

Is there someone who can help explain why when I try to paint on meshes with textures from my photogrammetry program; p.e. a mesh with a tiff texture; I get unpredictable results.

Sometimes the texture loads on import, sometimes not.

Sometimes the colors are negative. I guess I need to import in the paint room; then I can't sculpt anymore. I don't understand why I can't sculpt when I paint and viceversa.

Sometimes I push some buttons and I end up getting a textured model with a skin thickness and I can paint and sculpt. But I cannot repeat what I have done; it's like a sheep-path that I can't remember.

Please explain? You see I got some results yesterday, but I don't know how.