Anyone familiar with the history of garden hoses and how they came to be mostly green? Did they start out that way?
So they blend in with the grass color. Unless you don’t water your lawn, like me. Of course, since I don’t water, hose color is moot. 
That answer makes sense, but sometimes it’s not a good thing to have a hose that is hard to distinguish from the grass, such as when you are running around (you might trip), or when you are mowing (you’ll have to buy a new hose.)
Aaron
If they looked like rattlesnakes then you would notice them and not trip or mow over them. It would also keep strangers out of your yard.
It’s not easy being green.
– My Garden Hose
It wasn’t until just now that I decided I must have a bright orange water hose.
Hoses are green so they WILL blend in to the grass so well that you WILL run over them with a mower so that you MUST buy another one, that is green, and you WILL run over it again, so you MUST buy another one, ad infinitum.
If they were day-glo orange, you’d only ever buy one, and the Hose companies would go out of business.
That is all.
There are fussy people in the world who would object to having actual grass stains on their hoses. Green minimizes their complaints. There are others (similar to the Lexus owner of a few years back who sued the state of Indiana for having a license plate that “clashed” with his car’s paint) who would prefer that the hose simply disappeared into the yard, rather than clashing with their exterior decor.
I’ll bet that if you used a black garden hose you would notice that it is better camoflouged in a lawn that a green hose.
Green hoses are dyed green because the lawn is green. They are shiny enough to be noticed however.
But try a black hose if you are trying to hide the hose in the lawn. The black is flat, not glossy, and it really doesn’t show up well in the grass.
I have a black garden hose, and it’s taken me two summers and countless scalds (not to mention enough cursing to blister the neighbors’ ears) to remember the effect that sun beating down for hours on a black garden hose will have on any water standing in it.
Hoses are green so you can turn the water on real hard and pretend that it’s a big jungle snake.