Thanks guys. Wojtek and Andy, spraying the sand yellow over the green would definitely be quicker and easier but, as discussed above, looking at the reference pictures I don't think this is the way it was done in real life so I'm thinking to do it green over sand to replicate the camo pattern as realistically as possible. I'm concerned if I do the sand over green the sand will just turn out looking like mottles rather than the pattern you can see in the references.
Maybe I'll do another trial with sand over green while I'm doing my next session to get white onto the airframe...