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Some growers will try to get their plant to artificially display colors by shocking the plant with very cold temps. The reason this can work is because it disrupts the uptake of phosphorus, and in some plants this will cause the upper leaves under the light (and the adjacent buds) to turn red or purple. This does not work with all strains. The type of reddening or purpling caused by low temps will often result in smaller yields becasue non-green leaves are not good at photosynthesis, and the plant needs phosphorus to produce buds properly.
The leaves of this plant turned purple over night after that plant was exposed to chilly temps.
Cold temps may help the plant purple up, but it will also likely slow plant growth and hinder many aspects of a flower's development. It's generally not recommended for growers to turn down the temps just to achieve colored buds, especially at the beginning of the flowering stage – you may be doing more harm than good! If you are looking for purple buds, you'll get the best results by finding purple genetics that look/smell/taste the way you like. A "true" purple cannabis phenotype with turn purple no matter what you do in the environment.
If you do plan to use cold temps to shock your plant into producing colors, at least to wait until just before harvest so you don't accidentally reduce your yields. It's also important to note that just because the buds have turned red or purple, it has nothing to do with their final potency. Purple buds are not any more potent than green buds.
Leaves near the colas may turn pretty colors after being exposed to dark temps, but it also slows down growth and inhibits proper bud production. You need green healthy leaves for the best bud production in the flowering stage, not purple and yellow leaves that don't have the phosphorus they need to complete photosynthesis and make energy to product buds. A leaf like the following looks neat but will deliver very little energy to your plants.