I have a light coloured felt hat that I gave a naptha bath too but steaming it left residual water marks that lead me to believe the naptha did not get out all the dirt. So rather than re bathe it I tried the white pads on the water marks and it removed them without taking much, if any, of the felt.
Depending on what process you used with the naptha, that isn't surprising. If you did the soak the whole hat in naptha and maybe swirl it around once or twice method, then I'd expect that. I've said a few times, that once the dirt comes off the felt and into the naptha, swirling it around is just going to possibly redeposit the dirt back onto the felt. That process also uses a lot of naptha.