Multi-colour novelties in Yuzu are currently a bit complicated, each colour is a different layer icon:

There currently isn’t a simpler way to set them up, though improvements here are on Yuzu’s roadmap! For now, the easiest way to change a key into a multi-colour novelty is to copy and paste from an existing keycap:

and that will copy everything you need over:

The premade kits are set up with plenty of novelties to copy from. If you started from a keyboard, there should be at least a few, but adding the ones that aren’t there already is difficult. If you are unable to find or add a novelty you want, please feel free to ask in my Discord for help!