Shrimp have multiple color recptors because their brains are too primitive/rudimentary to combine input from more than a single receptor into a composite color. The result is that 12 colors (or however many receptors it is) is the total number of colors they can see.
So can we. It's also like to point out that seeing polarized light is different from being able to tell that it is polarized. Which we can still do, just not as well