The immediate mode UI library (ZUI) that the developer created, and which ArmorPaint is built on, is also really good. I don’t think Kha or ZUI get enough recognition, and I find both of them extremely elegant and pleasant to use. It might also be surprising to know that besides a few files for specifying constants and enums, ZUI is a single file library with only around 2100 lines of code.
https://github.com/armory3d/zui
https://github.com/Kode/Kha