Those roots are 'designed' to absorb water from the air, so when it rains or is very humid they get water for the plant that way. The outer surface absorbs water and turns green.
Here are a couple of pictures of some of mine - slightly green even though I haven't watered them in days. But the humidity is SO high here that unless they're out in the sun they don't dry out and turn back to silver/gray very often.
But my Vandas just got sprayed thoroughly with the hose 15 minutes ago, and their roots were all green, but have dried out because they're out in the blazing hot sun. There are still some that are greenish, down low in the shade of the plants.
So don't be surprised if the roots turn back to gray pretty fast if the humidity is low in your house.
My daughter in Utah - desert conditions - grows her mini-Phals in little glass globes with the leaves hanging out the openings. Hmm, let's see if I can find a picture of one. This helps keep the roots more moist for those poor little orchids growing in a climate that really isn't suited to them. She's had these growing and blooming for several years now.