The doppler effect can cause visible changes in visible light (if that makes any sense) which would be called red shifts and blue shifts.
One problem here: consider the normal case of a doppler effect problem, for example, an ambulance is speeding quickly towards you, a stationary observer, at 30 meters per second. You plug and chug into the doppler equation and figure out that the result is that the frequency of the ambulance's siren is higher for the stationary observer than it is for someone inside the ambulance. The difference in frequency is big enough that you can actually notice this example in real life. The reason is that for sound, the difference in velocity between source and observer is substantial relative to the normal speed of sound (343 m/s in a vacuum). Going back to our example, 30 m/s is substantial relative to 343 m/s.
However, when you move to visible light, the speed of light is huge (3x10^8 m/s). In order to get any realistically observable shift in the color of light, the relative velocity at which the source/observer are approaching or moving away from one another has to be huge as well -- big enough to represent some significant fraction of the speed of light. For this reason, the doppler effect causing shifts in visible light isn't something you really have to worry about for the MCAT, in my opinion. It's a phenomenon only observable on an astronomical scale, when you consider the speed of galaxies and all that.
However, when it does happen, all the terms "red shift" and "blue shift" mean is that the wavelength of light is becoming shorter (in the case of a blue shift) or longer (in the case of a red shift). Red and blue are used as markers of the two extremes of the visible spectrum.
In the same way, scientists doing UV/Vis spectroscopy can use the terms "blue shift" (or hypsochromatic shift) and "red shift" (or bathochromatic shift) in the same way that they use "upfield" or "downfield" when talking about NMRs. For example, the more conjugated double bonds in a conjugated system, the longer the wavelength of maximum absorption on a UV spec analysis of the compound. So you could say adding more conjugated bonds to a system causes the absorbance of the compound to be red shifted, since the maximum absorbance peak moves towards longer wavelengths.
All of the above has nothing to do with absorption/emission. When you perceive an object as green, that's because it's absorbing red light and reflecting green light due to its absorption spectrum. This is a totally different concept.
The only place I can see the two concepts ever coming near one another is in UV/Vis spectroscopy. I explained how the terms red/blue shift can be used in UV/Vis spectroscopy. When you have highly conjugated compounds their absorption maxima are in the visible range and thus you can use the complementary color rule to figure out what color you will perceive the compound as (e.g. chlorophyll's conjugated system absorbs at a maximum wavelength which is that of red light, so we see it as green). Even within UV/Vis though, the two concepts aren't really related in any meaningful way, they can just both be applied there.
