Adding Color to Leather

Tooling leather comes naked. No color, no finishes.

It's essential to the process of tooling, as the leather has to be able to absorb water freely and evenly and any dyes or finishes impair that ability.
Dyes and color come after all the tooling is done and there are a variety of products and tools for getting naked leather dressed up. I have my favorites -- the ones I find most faithful after years of trial and error.
For making leather black or brown I prefer a penetrating oil-based dye. It gives superior color and evenness, and the colors do not have a tendency to bleed or rub off.
This dye has very hazardous fumes; it's combustible and toxic, so adequate ventilation is essential to your health and well-being. If you don't have a strong exhaust fan specially rated for such things, then I recommend you dye outside whenever possible.
If your yard is 3 feet deep in snow like mine, then open the windows and employ a fan and a protective mask. Seriously! Use gloves, too -- this dye will penetrate your skin as readily as leather.
I like wool daubers for application on small projects. For larger items I prefer a sponge brush. I advise against an air-brush without a special set-up for exhausting the atomized combustible particles. I do 2 coats of color for complete coverage. It's finished off with an edge-coating, an application of a conditioner and a water-resistant beeswax finish.

Sometimes black or brown won't complete the picture -- sometimes a bit of color is required!

I use ceramic-grade acrylic color. It bonds well to the leather -- better than some of the leather-specific acrylics! The variety of colors available is impressive and it's readily available at most craft stores. My preferred brand is prominently displayed in the photo ;-)
It mixes well, thins with water and applies easily with a brush. It has no toxic fumes, which is nice!

Once it dries completely a protective edge-coat is run around the outer beveled edge.

Then a highlighting product is applied generously, taking care to work it into the impressions.

Once the excess is wiped away, the remaining highlighter mellows the background and makes the detail pop! To finish my colored leather I use a coat of conditioner and then a spray silicon sealant.

Don't you just love color?