The best material that comes to my mind is Window Color, an acrylic based liquid paint used to create faux stained glass ornaments. Alternatively, acrylic paints designed to paint directly on glass work as well. It's important that the paint is formulated to stick to glass and stay flexible after drying.
The advantages of Window Color and glass paints are:
- It stays very flexible after drying. Whereas hot melt glue is rather flexible, most other paints will chip off.
- Colors that are vibrant and translucent are readily available.
- Paints mix easily.
- It's relatively cheap.
It should stick to hot melt glue, but I have no first-hand experience and my old Window Color paints are a rubbery, dried mess. You might have to cover the entire glue flame with a thick coat of Window Color to make sure it doesn't separate from the glue.
I did a test comparing acrylic glass paints with regular acrylic paints:
- Left (foreground): Hot melt glue painted with acrylic glass paint. The paint drew back from the glue in small places. It's translucent and vibrant. I only had yellow and orange paints, it would look better with red in the mix.
- Left (background): One bit of unpainted hot melt glue to demonstrate the opacity of the base material. My glue isn't very transparent.
- Right: Hot melt glue painted with normal acrylic hobby paint. The paint drew back from the glue instantly and only stuck to it in rough patches. The paint itself is also opaque. I stopped this test after applying yellow paint because anything more would have been a waste.
All other ideas I had for paints (common acrylic paints, alcohol markers) have the disadvantage that they don't stick well to hot melt glue. Something in the formula for glass paints makes them stick to hot melt glue as well.
Oh, and let me tell you how utterly horrible my "flames" look... If you really want to create something like in your question, you'll need a glue gun with adjustable temperature or work in a very cold environment. I barely managed to make 3 spikes stick up, not to mention add any flame-like details.