Wrinkles fade out when retopologizing When should I sculpt the wrinkles,texture?Before or after retopo?And if after, how do I add wrinkles if the mesh is retopologized? And can I texture paint the mesh if it is made by triangles and the retopo isn't done?

Thanks!
The big wrinkles should be sculpted and the small ones should be painted after the retopology and after baking the normals.
If your model is meant to be rendered within Blender, you can restore lost detail on the retopologized model by adding a multiresolution modifier and a shrinkwrap modifier that you apply (only the shrinkwrap). Note that you have to apply it while the shrinkwrap is below the multires, otherwise it doesn't work.

Painting on a triangle mesh is a bad idea.