It looks like it's probably some sort of vinyl, but here are a few more questions just to be sure.
1. The part on the right side on your image folds over to attach the holster to the belt, right? Is the material kinda flexible?
2. And are the stitches along the edges actual stitches or are they just indents?
If yes, then I would recommend Angelus Brand Acrylic Leather Paint and a bottle of Angelus Acrylic Finisher to apply over the coat of paint once it has dried. Although Angelus paint is marketed for leather, it adheres very well to vinyl and pleather (faux leather).
It comes in many different colors and has very good coverage.
To achieve the best results with this brand of paint, I suggest the following: 1. Wiping down the holster with a little rubbing alcohol to remove any dust before you paint. (If there is different paint on the holster, use some acetone to remove it.)
2. Painting with a foam brush to minimize the appearance of streaks.
3. **It may also be a good idea to test out the paint on a piece that you are not planning on using for your cosplay, just to make sure it will adhere correctly. I don't think you have to worry about this, but sometimes if the surface you are trying to paint has a silicone finish to it, the paint won't stick right. Angelus makes a product to remove silicone finishes as well.