Depends on what effect I'm going for, but usually it's worn leather, so I start with a tan base and then add splotches of black or brown. For saddle leather and other natural leathers, I choose from Burnt Umber, Burnt Sienna, Mars Orange, Mars Yellow, Vandyke Brown and Gold Ochre to come up with the right mix. I think that oils dry with a more "leathery" finish, so I use them for unfinished and undyed leather. I sometimes use oils for black and white leather too.
I have no clue what Tamiya colors to use, but there's really no official leather mix. No magic bullet. It's pretty subjective. Look at what colors you have and compare them to stuff you have around the house that are made of leather, or, if you have a picture or a real smaple of the subject you're trying to paint, then match it to that. Are the basic colors you have too red? Too green? To yellow? Too dark, or light? It's a simple matter to flip them over.