system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we actually know what colours other animals can or can't see?
gpt
The rods and cones in your eyes contain dyes, which are the chemicals that do the light sensing.  A dye is just a molecule that absorbs a very specific frequency of light.  This makes them a particular color, and you can literally just measure the light reflected by these dyes to see what color they absorb.  

These molecules also bleach as they absorb light (which is how they adapt to brighter light), so you can also see what color light you have to shine on them to make them change color.