Final answer:
Violet light has a higher frequency and greater photon energy than red light, leading to a potentially shortened processing time when using a lightener on gray hair.
Step-by-step explanation:
The question relates to the colors that influence processing time when using a lightener on gray hair, with a focus on identifying the color with the shortest wavelength. Violet light is the answer because it resides on the opposite end of the spectrum compared to red light and has a higher frequency and greater photon energy. This higher energy level makes blue-violet light more potent in processes like photographic emulsions and, in the context of hair lightening, can contribute to a quicker processing time.
It's important to note that while violet light has higher photon energies, which is beneficial in some processes, it can cause more damage to sensitive molecules like those found in dyes and DNA. Such information underscores the importance of understanding light and color physics when applying it in practical scenarios such as hair lightening.