simple:Red

Red is a color at the lowest frequencies of light discernible by the human eye. Red light has a wavelength of roughly 700 nm. Oxygenated blood is red due to the presence of hemoglobin.

On a browser that supports visual formatting in Cascading Style Sheets, the following box should appear in this color:

Lower frequencies are called infrared, or far red. 

Usage, symbolism, colloquial expressions