ANNUALS
Anybody who pays attention to gardening has heard about annuals. Annuals are plants whose natural life cycle lasts about a year. They grow from a seed, mature, reproduce, and die within a year. Annual plants are differentiated from biennial plants (lives 2 years) and perennial plants (lives longer than 2 years).
Cannabis is considered an annual plant because it will follow the annual cycle under normal conditions. And like many annual plants, cannabis will switch between different stages of its life cycle depending on the lighting conditions.
In the wild, cannabis will grow from seed in late winter/early spring, enter vegetative state in late spring, maintain vegetative state through summer, and enter flowering state in fall before dying in late fall/early winter.
Spring light is generally bluish-white.
Summer light is generally white.
Fall light is generally red/yellow.
Cannabis has evolved to respond to these color temperatures most, which explains why these lighting conditions are ideal. Some growers I have spoken to also inject extra blue and UV light during the final stages of flowering for a last push before the plant expires.
With all that said, many growers still choose to stick to a single lighting regimen for their plants. Since photoperiod and scotoperiod determine the transition points for cannabis, growers will often elect to buy the bulbs that exhibit the highest PPFD numbers and simply change lighting schedules. Large scale grows will often choose this method for practical reasons. This method works, but is less efficient than a gradual transition in color temperature.