Honestly use a hair paint on your beard that way you can also stiple the colour onto your skin as well in the beards.
I would paint you heard first then take a small brush to the edges with the green.
If your hair is the right length you can also pre style then paint your hair as well.
With the hair paint I recomend styleing the hair first with a hair glue because the produt dries stiff (got2b Glued spiking Glue).
This product you can also mix in a small amount of the paint first to tint it as well before. Set the product dry with a hairdryer to speed up the process then paint.