Figure 3. Three dimensional representations of various discs in color space.

Colors of the blue background and of the various grey discs, shown as vectors representing the relative excitations of the red, green and blue photoreceptor channels. The UV excitation is not depicted, as it is very low. The blue vector represents the blue background. The green vectors represent the various grey discs, except for one grey disc (Dreadnought Grey), which is shown in red. The vectors for all of the grey discs have almost identical directions, indicating that the hues of the grey discs (as perceived by the birds) are all very similar.