Paint does not always adhere well to leather. In order to stick and to last, the paint needs to penetrate into the leather - the deeper the better.
Here are some tips I have learned on how to prepare leather for successful painting.
Before applying paint to leather, clean the surface thoroughly. Wipe it with a water-dampened cloth or a small amount of mild soap and water. I also have found that using an old toothbrush works well.
Suede leather usually holds paint well, but smooth, shiny leather often has a wax or oil finish that prevents paint from adhering. Be sure that you have taken the shine off portions of the leather that will be painted. This will help the paint to bind.
Wipe with rubbing alcohol to remove surface oils and any protective coatings from the areas where you will be applying paint. (I also have read that denatured alcohol and acetone are other good options.)
Then lightly sand the areas that will be painted, using fine sandpaper or emery cloth. Besides helping to remove the coating, sanding also will "rough up" and "raise a tooth" in the painting surface just enough to make a big difference in how well the paint adheres. But be careful not to apply so much pressure that you damage the leather. Easy does it!
I hope these tips on how to paint leather have been helpful to you.
See Paint Leather Jackets to watch a series of short tutorial videos by talented artist Kristine Mueller.
For information on using opaque paint markers on leather, Acrylic and Oil Based Paint Marker page.
Two of my projects that involve painting leather are Painted Baby Shoes and Winter Arts and Crafts: Alpine Skates.