They can look at the retina under a microscope and see that dogs’s eyes only have two types of cones, whereas humans have three. This means dogs can only break the perceived light into two bands instead of three, giving them a smaller range of colour perception. This can then be tested using coloured indicators and food to determine whether the dog can distinguish between certain colours.
Conversely, dogs have more rods than we do, which gives them better night vision.
Always wondered how the “experts” came up with that theory.
It’s not like the dog can communicate the information to them.
Still doesn’t explain how the experts come to the conclusion that dogs are colour blind 😜