well, arguably, navy versus cornflower would handily cover pedantic differentiation, as needed.
put another way, one can easily counter that the flag and the sky handily share similar saturation of the same hue, differentiated only by the value of darkness.
take it one step further, and the sky at dusk will drop its brightness, and even if only for a moment, match the flag’s deeper blue, until the sun completely sets and the night sky becomes black, when not contaminated by light pollution.
put another way, one can easily counter that the flag and the sky handily share similar saturation of the same hue, differentiated only by the value of darkness.
take it one step further, and the sky at dusk will drop its brightness, and even if only for a moment, match the flag’s deeper blue, until the sun completely sets and the night sky becomes black, when not contaminated by light pollution.