I noticed that the color of some of the tag names that I created are now green. What caused the tag names to turn green?
By default, tags or labels are coloured green. Do you mean that you had set a different colour for these tags and it was reverted to green? You can also correct them by going to the "Label" tab and change the colour there.
I noticed that on my outlook not all of them were green. But on my iPhone they were all green. I don't need different colored tags. It's fine.