Points for you if you already understand this physics-based bumper sticker. The effect is called blue shift. From Wikipedia:
Blue shift is the shortening of a transmitted signal's wavelength, and/or an increase in its frequency, due to the Doppler Effect, which indicates that the object is moving toward the observer. The name comes from the fact that the shorter-wavelength end of the optical spectrum is the blue (or violet) end, hence, when visible light is compacted in wavelength, it is shifted towards the "blue" end of the spectrum. Since the longer-wavelength end of the visible electromagnetic spectrum is red, the opposite effect, of a lengthening of a signal's wavelength, is referred to as redshifting.
While the terms "redshifting" and "blueshifting" imply significantly redder or bluer light, only the most distant galaxies and those moving at speeds far above average emit light that arrives with perceptible red or blue tinges. For the most part, shifting is not a visible phenomenon.