We see it blue, but how is it seen by his fellows? - Some facts about avian vision
No way to know really. Although we tend to be somewhat self-satisfied with our own color vision, it is not particularly well developed when compared with that of most vertebrates. The color vision of most humans relies on three types of retinal cone photoreceptors, all of them neurally integrated in the assessment of spectral radiances and thus in the perception of color, our colors are mapped in three-dimensional color space (we are “trichromatic”).
In contrast, most birds have four types of cone involved in their color vision and are likely to be tetrachromatic. The consequence of four cone pigments, and tetrachromacy in particular, is that birds see the world differently from humans and in a way for which it is hard to compensate because we simply lack the neural machinery.
There are also other additional physiological differences that limit our appreciation of a bird’s view of the world. First, most of the retinal cones of birds contain oil droplets with high carotenoid content that act as spectral filters and modify the spectral sensitivities of the cones. Second, birds are sensitive to ultraviolet (UV) wavelengths, whereas humans are not.
The differences between human and avian vision mean that, for many purposes, human vision, or standards derived from human psychophysics, are inappropriate for studying bird visual behavior.
In the case of the ‘bluest’ birds, those that have the highest percentage of blue feathers on the body, such as the Black-naped Monarch (Hypothymis azurea), it is known that these ornamental feathers reflect light maximally at the shortest wavelengths (UV), with the greatest intensity and the greatest contrast.
References:  - 
Photo credit: ©Henry Koh | Locality: Kaen Krachan National Park, Thailand (2013)