Things have a way of not changing if there is no reason to.
I have to think the green color was used because so many other darker colors contrast with it nicely. For safety reasons I wouldn't want employees or personnel making a critical mistake because they couldn't see the right handle, gauge, or switch due to lack of color contrast. It got the job done and provided contrast, making things easy to see - all the more reason to continue to use it.
Then again, it may have just been cheap paint that got the job done.