Leather has a unique texture to it
The way I create the look of leather. I do a pointalism or blotching technique.
I have to basically work in layers of colors.

I first add the light greys and let that dry.
Then I increase the black to work in the shadows.

Note: My black color is made up of 100% Cyan, 100% Magenta, 100% Yellow, and finally 100% Black
I get this color scheme from all those years of working in the printing industry.
You get a deaper black color when you mix CMYK together.