well the original 青comes from a dye called 靛青(indigo-blue),later 青 can be used for describing those colors:
1. blue:青天means blue sky;
2. indigo blue:靛青
3. deep green:青草;青山;
4.black:青衫(black clothes)
I think 绿水 is easy to understand,:D
river around the mountain if not polluted always shows green color,:D