Airbrushing Accents

Haven't tried it myself necessarily, but it might work.

I think I actually have heard of people spraying an accent colour onto model trees or bushes so it's a bit lighter on top and darker below; I'm not sure it would look as good to do the reverse, or to try to completely change the entire colour, as it may not penetrate all the way down in bushes and folage clusters. 

I typically blend several similar colours of ground foam and foliage together for some general colour variation; airbrushing some accent colours might additionally blend things together or add more variation.

I would perhaps create a test piece of scenery, about 1-2 square feet, on a scrap piece of foam or plywood etc. Do part of it lighter, and part darker, and test out the concept to see how well it works.

Joe recommends it

Joe recommends painting any foliage with a couple of colors.  He has medium greens for the bulk of the foliage and then highlights the tops with yellow to simulate sunlight on the tops.  I've also seen others on this forum use the same type of technique on grass coverage to get the correct tone required.  So this is generally done by a lot of folks...

I have used an air brush

I have used an air brush using acrylic paint sevral times just to bring the foliage back to life after a year or so or after a construction project. This included trees, brush, rights of ways, etc. It is easier than vacuuming.

Steve