Pier Paolo,
I wish I could offer a definitive answer, but I can't. I happen to be in the midst of painting some Gurkhas and having some more painted for me, so I've been researching the same issue for the past few months. I've found contradictory written evidence in my go-to authoritative source: Carman's "Uniforms of the Indian Army, Infantry." There are separate entries for each Gurkha regiment, and some include quotes from their dress manuals. With regard to the pillbox -- aka: Kilmarnock -- cap, sometimes it says "green" other times it says "black"; for one particular regiment he says that caps described in the manual as "black" were in fact rifle green. If there's a particular regiment you want to recreate, let me know here and I'll be happy to check what -- if anything -- Carman has to say about their caps, and to let you know. I myself have gone with the rifle green, but I'm first to admit it's not based on certainty.