Answer: Violet

Visible light can be measured in terms of both wavelength (the distance between the crests of the waves of photons that comprise the color) or in terms of frequency (the rate at which waves pass a given point). The color frequencies in the visible light spectrum are measured in terahertz (THz) and range in frequency from 405-788 THz.

Among all pure visible colors (colors which are comprised of a single frequency and not a mixture of visible light frequencies), the color violet has the highest frequency with shades of violet ranging from 666-788 THz. Once you push beyond 788 THz on the spectrum, you enter the territory occupied by ultraviolet, a higher frequency electromagnetic radiation invisible (outside of a few exceptions) to the human eye but visible to some insects, mammals, and birds.

Ultraviolet is higher in energy than visible light and capable of causing chemical reactions like sunburns, triggering fluorescence in some substances, and fading the color of fabrics, plastics, and other materials left exposed to it for extended periods of time.