https://www.dickblick.com/products/gallery-glass-paint/

Colors can be mixed. And multiple coats will give a more saturated color. A little goes a pretty long way, don't let the small bottles scare you.

Might be cheaper to buy it as a package of colors:

https://www.amazon.com/Gallery-Glass-PROMOGGII-2-ounce-Selling/dp/B00ATJSDGK