system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know what colors are visible to other animals and why are the visual spectrums so diverse?

I have heard the old addage that dogs are colorblind, but what do people really mean when they say that? Because of science we know that dogs have a very limited visual range, yet butterflies can see more colors, and don't even get started on Mantis shrimp.  As if they were not already the most badass creature on earth, they get to have godlike perception as well? 

I know there must be a sciency way that people have used to figure out the visual perception of creatures... But for the life of me it does not make sense in my head.  As humans we can use our imagination, technology and bad CGI to see the world as some of these creatures do... But how did we find out what they can see to begin with?  I am especially interested in how we know what dogs see, and how the heck that was figured out... But I would also like to hear about other creatures eyesight capabilities and limitations and how we as humans came to those conclusions.
gpt
We can know because vision works in a similar way in all animals (including humans). 

For humans, our eyes, have two kinds of vision cells. One kind is called 'cone cells'.  Cone cells are in charge of detecting colour. Cones themselves are further divided into three types depending on what wavelength (colour) of light they absorb. One type absorbs red, one absorbs blue, and one green. (Technically all three absorb all colours, but peak in red, blue, and green). Every other colour we see comes from these three colours. Our eyes and brain work together to process and create the remaining colours. Since we have three colours we detect, humans are called trichromatic.

Dogs are dichromatic. Which means they do not have three different types of cones. They only have two. They have vision equivalent to red-green colour blindness in humans. This means for them red and green look the same. In general, most mammals are dichromats.

Cool fact: Some humans (mostly women) have four different types of cones which means they can distinguish and see colours normal humans can't.