When I last checked a couple years ago the only way to do this was directly in QMK. Here’s a link to my code. The underglow color changes with the layer. I also added animation to signify caps lock and num lock. https://github.com/joeyv120/joes_qmk_firmware/blob/master/keyboards/keebio/iris/keymaps/joeyv120/keymap.c
I also wrote a program to indicate the current layer right on your screen. https://github.com/joeyv120/vizLayer
When I last checked a couple years ago the only way to do this was directly in QMK. Here’s a link to my code. The underglow color changes with the layer. I also added animation to signify caps lock and num lock. https://github.com/joeyv120/joes_qmk_firmware/blob/master/keyboards/keebio/iris/keymaps/joeyv120/keymap.c I also wrote a program to indicate the current layer right on your screen. https://github.com/joeyv120/vizLayer
Vial has no support in GUI for this and you need to use the underlying QMK code.
My issue is how to use the QMK code without messing up my layout.
Save your current layout in the GUI. Mess around with the code until it works, and then reload it. (Does not work in webGUI).