Yorkshire Terriers

i have a 3lb 2 year old male yorkie. his name is max. he has a brown rash on his lower stomach that looks like he is dirty with a few bigger bumps. when i give him a bath and scrub that area it does not come off, so that is why i am assuming its a rash. has anyone ever came across anything like this or know what it may be? he has had flea allergies in the past. it doesn't seem to bother him but i am just curious to what it could possibly be.

thanks so much!

I also have a male, Hank, who weighs about three and a half pounds. He has the same "rash type" on his stomach/belly. It almost looks like he has rolled around in d dirt. Also when I give him a bath, it does not come off. He does not have any bumps though. I don't know what it is, but I would like to know.