In the London shop, Edward Green will show you samples of green and petrol-blue leather, if that is what you desire. I must say, I was a bit shocked about the blue leather. (Lattanzi, yes...but EG? It's like some dowager duchess going punk.)
One of the shops that I frequent has a couple of Moreschi models in blue peccary. It's not quite as garish as you might imagine. One of the shoes that I want to do at some point is JL Paris's William in green Scotch grain. Not particularly tasteful, but fun. But Edward Green in either one? I don't think so.
Apart from that, as the final color and finish is applied to the completed shoe, they probably can match the color to a sample you specify. Have you received your special order "Stowe" already? Otherwise you still have time and you can get the shoes matched to your favorite socks.
I have indeed gotten my Stowes, so that's not an option. Besides, I would have to get different pairs to match each of my sock colors, and that would hardly be economical.