Okay I think the voice of the people has declared tap/hold/toggle, you could also have it be energy-color dependent which would make it less flexible, but more consistent, and make it less broken on helminth.
If you wanted to add complexity and synergy you could both buffs active at once, but have them be charged and then deplete over time. For instance you could stand in light for a X seconds, then after you leave maintain the damage up buff for a X*duration_modifier seconds. The player could produce their own light with prism, and their own dark with slight of hand to add some level of player control to this. This would be less consistent, make eclipse a worse helminth ability, possibly be more overpowered but in an annoying to play way, but I wanted to put it out there.
