Ice water

No 1st you need genetics to do so. Then you will require temperatures below 67 to start degrading chlorophyll, anything below 67 will negatively effect growth and should be avoided until ripening occurs unless color takes precedent over yield. Temps below 64 will also start to effect manganese uptake through the root system, which is a pretty big deal during flower. As far as I'm aware flushing does nothing for color. Wouldn't bother tbh.

Some people put ice or cold rain water. Putting water straight from tap will bring chlorines and lockouts, your plants will lose colours but won't develop too. Many growers do that mistake. .. Some strains just don't have it in them, just simple yellowing .. Nice girls ! All the Best !

Colder the water the more the plant will go into shock, not ideal in the root base. Colder temps "10-15c" in air bring out colours from the degrading chlorophyll. tuning purple. Use room temp water and bring temps down at night for some extra *POP* Good Luck!