1. Not really darker, but a different sort of hue right after. Lightens again after powdering though.
2. When the TPE doll gets dry, it will be a little lighter in color but typically returns once you re-hydrate.
3. What you may be seeing because of not frequent enough cleaning or environment excessively dusty etc is the dirt coming to the surface from the mineral oil application. It is a good practice to work the oil in and remove the cruddy oil off the doll. Then apply vaseline petroleum jelly lightly & let it absorb overnight.