Fanshobby aren’t like that, their figures are passion projects. Double Evil is a top example of this so I’d expect that they will stay faithful to the design. If you did want to paint it however usually Fanshobby use coloured plastic rather than paint as their toys are designed to be played with and look good on a shelf at the same time. I think one of those crystals that are translucent that lets the colour flow through like a prism would be the best choice. The Gundam teal you’ve mentioned would work too as would the LED. We know the super mode has LED in the head, I wonder if the core figure does? That would be amazing.