Guide Dogs and Green Vests in London
In London we've received a lot of, "If he's a real guide dog why isn't he wearing green?" First of all, blind people can & do style up their dogs. Second, guide dog schools exist all over the world with many variations. It's the training that makes a guide dog, not the outfit. Believe blind people.
A common harness is white with reflective material that looks neon green/yellow, or checkered red to represent a Deafblind human partner. Neon green vests for various assistance dogs is common, too. But again, it's the training that makes an assistance animal, not their color.