I've used a 50/50 glycerine/water solution to preserve plants and lichen for use in groundwork. The fresh plant matter or lichen is soaked in the solution for about 2 weeks, and then removed and placed on paper towels for several days to absorb the excess glyerine solution. The result is supple and realistic, although the color of the original material may change during the treatment. For example, much of my lichen is gold or orange instead of the original pale grey-green. Greenery may turn dark black-green or bronze in color. I expect the plants could be airbrushed with watercolors (glycerine is an ingredient in watercolors), or possibly acrylics, but I haven't yet tried this. I understand that plant matter treated this way is quite long-lasting, but I've only been doing it for a little over a year. The first stuff I preserved this way is still supple and springy, although some of the plants have faded a little in color.
Mike Szwarc