I might be missing something here, but regarding low kelvin not being favourable for photosynthesis, my dad's HPS lights in his greenhouse are what I expect is about 4,000k, almost red.

My point exactly!
So the title of the thread has Refugium in it and the goal of a fuge is to grow algae and other critters.

Red is favorable for photosynthesis, especially in plants than run on chlorophyll a because they are best able to use red and blue. so you can grow land plants on red light, blue light or best of all, a mix of red and blue (like those plant grow bulbs you see). Unforunately, humans see greens best so we can't appreciate the brightness of a plant gro bulb.

Bert