sir_isOanswered grow question 4 years ago Okay, here are some observations.
Autoflowers are genetically related to plants from shitty areas (environmental pressures), so they try flower as quickly as possible.
In shitty areas, often, you have cold and low light, short growing season, so the plants try to reproduce as fast as possible.
Due to that, they deplete themselves quicker (such as using phosphorus from numerous parts of the plants to flower).
Another thing is, anthocyanins (red to blue pigmentation, most often purple or even black) is related to what is essentially a combination of those factors (like strain adaptation to environment, nutrient availability, stress, low light, etc). So a plant that wants more heat and light or phosphorus (or is using a lot of it), will often turn purple or black. Phosphorus deficiency (relatively speaking) is most commonly related to that anthocyanin production, along with calcium and nitrogen, as those factors are very related (from what I've seen).