Q.My Kaffir Lime Tree Has Orange On The Underside Of The Leaves
It’s never had it before, it’s not powdery and it has lots of new growth.

Certified GKH Gardening Expert
Unfortunately, your photos did not come through. I am unable to diagnose the situation fully. I will say that this sounds like scale. This insect will devastate your tree if so. Here is an article that can help:
https://www.gardeningknowhow.com/edible/fruits/citrus/controlling-citrus-scale.htm
This article will help you with Kaffir lime trees:
https://www.gardeningknowhow.com/edible/fruits/lime/care-of-your-kaffir-lime-tree.htm
This collection of articles will offer more information on the care of citrus trees: