A dark basin is the reflex answer to “we want it to be a mirror”, and it works: kill the bottom and what you see is the sky. We have built plenty. But in a hot climate a dark 40 mm sheet is a solar collector with a very small thermal mass, and our model for the Lisbon courtyard put it at 41 °C by mid-afternoon in August — a chlorine demand you cannot dose against and a biofilm you can smell.
The thing people forget is that reflectance is dominated by viewing angle, not by what is underneath. Below about 15° from horizontal, Fresnel reflectance from a water surface climbs past 60 per cent and the bottom stops contributing. So the question is not what colour to make the basin — it is where people will stand.
At Salt Court we dropped the surround by 300 mm so that every approach reads the water at under 12°, then used a pale honed limestone bottom. From anywhere anyone actually walks, it is a mirror. Stand directly over it and you can see the stone, which several visitors have told the client they like. Peak water temperature in the first August was 29 °C.
This does not work everywhere. If the pool is seen from a balcony above, the geometry is against you and you are back to a dark bottom and a chilled bypass. But it is worth ten minutes with a section before anyone specifies black render.