Cistern
A cistern is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. To prevent leakage, the interior of the cistern is often lined with hydraulic plaster.
Read on WikipediaRainwater in a concrete tank, calm and unstirred.
Cistern is a light grey.
It can be described as light, pale, neutral and almost grey.
Set in black, because black reads best on it.
A cistern is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. To prevent leakage, the interior of the cistern is often lined with hydraulic plaster.
Read on WikipediaNoun