Humans have red green and blue photoreceptors allowing them to sense colours in the spectrum of about 400-700nm. Certain proteins allow for the extending of wavelength range in the RGB receptors, this however is only perceived as more shades of the colour in question. Additional photoreceptors are necessary to sense a new colour, explicitly separate from RGB.
The issue is that there seems to be no information detailing the the structure of mantis shrimp eyes, specifically regarding the the aspects of which are responsible for vision into the near infrared spectrum (roughly 700-1000nm).
What would the cones of an organism that saw exclusively in infrared look like? (assuming the organism had eyes like a human)
Would they be differently coloured or structurally different to our own?