Green water is just water filled with suspended micro green algae. U're likely to achieve this if you have high nitrates/phospate levels in the tank water coupled with high light levels. This promotes an algae bloom.

Green water is a double edge sword. There's a theory that these micro algae can promote the colours of the goldfish (or kois). Kois or goldfish that come out of these murky green pea soup water in lightly stocked outdoor ponds have great colour.

However, in an indoor tank where the majority of people have the penchant for over stocking with fish, coupled with the small volume of water (as compared to the extensive ponds), green water can quickly lead to disaster by suffocating your goldfish at night when the lights are off. (Process of photosynthesis doesn't happen at night and the algae compete with the goldfish for oxygen at night... goldfish always lose.... ie dead goldfish in the morning.)
Of course an airstone would help alleviate such losses.

Another problem with green water in small volumes of indoor tanks is also the pH fluctuation between night and day caused by the respiration and photosynthesis of the algae. This dramatic shift in pH may also not be best for your fish.

Lastly, would you like to have a tank of green cloudy water sitting in your living room with only flashes of some orange seeing thru once in a while?

Green water is great to restore goldfish colours, outdoors in very lowly stocked ponds, but not the prettiest sight in your living room.