can anyone suggest a good way to replicate this sceme without leaving a buildup of paint on the green areas due to hand brushing?

If you have an airbrush, paint thelighter grey/silver first, let it dry thoroughly. Then paint out the lighter area with Maskol or Art masking fluid making sure you cover all, then spray the green, in thin coats, removing the maskol once its dry.

My thoughts on the way anyhow, practice on some scrap first...I would

Keith