How the Blue gene work in chickens. Doesn't matter if it is on a solid colored bird, a silver based bird (grey families of gamefowl) or red based bird. This also applies to patterns such as spangled, laced, mottled, etc. In gamefowl blue is often used as part of a signifier in a bloodline's name, but the birds themselves do not always come blue in color. They could have been bred to become pure blues, which is why they appear to have white or red and white plumage, or had the blue gene lost out of them which is why they look black and grey or black and red.
