I think its because historically, Muslims/mughals adopted the colour green but the important colour for Hindu's is Red. I'm pretty sure the British wore red as well. However Guru Gobind Singh Ji Maharaj selected the colour Blue. This had practical reasons too since in battle if a Singh was injured the blood would just look like sweat (Thats what I was told from a Katha) so therefore would be less targeted. I don't know much about this but this is just my guess.
Bhull Chuk Maaf