Despite their 12 photoreceptors, mantis shrimps are worse at telling apart different colours than humans, honeybees and butterflies.

We see colours by making comparisons between our three receptors. By comparing the outputs of the red and green receptors, we can tell the difference between reds and greens. And by comparing their combined output against that of the blue receptors, we can discriminate between blues and yellows. This is called the “colour opponent process” and it’s what every colour-sighted animal does.

Their working hypothesis is that the mantis shrimps analyse the outputs from all of their 12 receptors at once. Rather than making comparisons between those receptors, they pass the entire pattern of outputs onto the brain, without any processing.

Marshall suggests that this way of dealing with colour should be much faster than ours, since there is no need to send the photoreceptors’ signals through any intermediary neurons.

And they only have a small brain. “A mantis shrimp only has a fraction of our cortical processing power, yet it handles 4 times more input,” says Nicholas Roberts from the University of Bristol.

https://archive.ph/nmPyC

tl;dr: It's my hypothesis that the clam overloads the stomatopod's tiny brain with a massive amount of input.