Let's consider an air-glass interface. When light passes from the air to the glass its frequency stays the same, but the velocity and wavelength change. Now the problem starts. If the colour stays the same then, as I was taught in school, the frequency as well as wavelength should stay the same, because they both equally determine the colour of light. But just one of them is constant! So what determines the colour of light: frequency or wavelength? Or maybe there is a mistake in my way of thinking, namely the spectrum changes when the light changes the medium? Or maybe something else?