What tamiya colors do I mix to get "leather"?

Oooooh, good point. Grab some Vellajo paints if you prefer Acrylics. They're designed specifically for figures.

Further, if you're moving into figures. you'll want to move away from tamiya, which is just not brush friendly. Any of the other modeling acrylics will work far better.

One thing I have found and that adds to what plymonkey said.

You are best of mixing your own colours and for I found most modeling paints less convenient that artists paints like Liquitex, etc.
Acutally as I said in another thread I am busy ditching most of my modeling paints and use mostly artists acrylic paints as I can mix any colour I want.
Plus, they safe on storage in a major way and end up waaay cheaper.

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.

I normally start of with a color close to original leather and than add the tan(colour) to it afterwards.

This way you can let the untanned leather show through, said that closes to 1/35 I have done it in was 1/12 and 1/24 figures.

What shade of leather are you looking for? Lether can be any color. Varies in shade from a light brown to deep black as well. All these can be mixed or come already in the color by someone, depends on what you are looking for and what you will be painting

We need a little more info to help you.