See hotkey R
Indoors Dragonfly UI (Entangling roots spell not usable), hard to see

Indoors Default UI (Entangling roots spell not usable), easy to see

Outdoors Default UI

Outdoors Dragonflight UI

Is the first picture with complete colour desaturating intentional?

Kind of intentional, yes. But I'll look into it again, as the default one seems a bit nicer with some color left.

The code was

 if not isUsable then
        mask:SetVertexColor(0.4, 0.4, 0.4, 1.0)      
        mask:SetDesaturated(true)
        ...

I changed the VertexColor back to mask:SetVertexColor(1.0, 1.0, 1.0, 1.0). This should make the button like default when the button is just not usable; out of range is still changed.