Dogs see in black and white
The old claim turned out to be wrong and was rejected by evidence.
TaughtDogs are completely colorblind. Their world is a grayscale film.
NowDogs have two types of cone cell and see blues and yellows. They cannot distinguish red from green, which is why a red ball vanishes into green grass for them.
What actually happened
The claim seems to trace to Will Judy, a dog writer who asserted it in 1937 with no evidence, and it hardened into common knowledge. Jay Neitz and colleagues at UC Santa Barbara tested it directly in the late 1980s with behavioral experiments, training dogs to select colored panels for a treat.
Dogs turned out to be dichromats: they possess cones sensitive to roughly blue-violet and yellow-green wavelengths, giving them color vision comparable to a human with red-green color blindness. The practical consequence is worth knowing if you own one. That bright red or orange toy you throw onto a lawn is, from the dog's point of view, a dull yellowish object on a dull yellowish background. Buy blue toys.
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Who was taught this
Still standard through 2005, so anyone who finished school between 1950 and 1989 learned the earlier version.