I had the same question a few years ago, I ended up using the Blagdon paint in black, and the reason being that the green and the blue will inevitably turn dark green/black anyway. It won’t take long to get a dark layer of algae over all the surfaces below the water line. So don’t be disheartened if you paint it blue and it only lasts a few months
My pond, 2500g concrete block, render and pond paint, 1 bottom drain,15,000 litre hailea from vortex, 15,000 litre hailea from mid water feed and skimmer, 3kw elecro heater then all 30,000 litres to four tier CB shower.