Fig. 3: Tetrachromatic avian color space.
From: Hummingbird plumage color diversity exceeds the known gamut of all other birds
The avian perception of a reflectance spectrum is represented as a point in color space determined by the relative stimulation of the ultraviolet or violet (uv/v), blue (s), green (m), and red (l) retinal cones. The center of the tetrahedron is the achromatic origin, which represents equal cone stimulation– either white, black, or gray. Each color point is defined by the angles θ and φ, which define the hue, and the distance r, which defines the chroma or saturation (reproduced from Stoddard and Prum 2008).
