How can Giant squid have $20,000 mm^{-2}$ photoreceptor cells, but no cones?
First squid lack both cones and rods, they evolved their eyes independently, the cells they use most closely resemble rods but they are by no means identical.

In vertebrates there are two types of cells rods and cones both of which can be used to see movement (movement is just detecting changes in the visual field). rods are better at it becasue they are smaller so they can be packed tighter, thus they can detect smaller changes, but that is by no means their primary function. Rods detect luminosity, that is the intensity of light, but react to light from all parts of the visual spectrum. 

There is a lot of debate about how much color squid and octopi can see. behaviorally they appear to recognize color, but they don't have differentiated cells or opsins to detect them. 

I appears they have stumbled on a completely different way to achieve color vision, they appear to use weird shaped pupils and chromatic aberration. 
I suggest reading the linked paper as you have to dive pretty deep into the physics of optics to understand. Basically different colors of light are bend by slightly different amounts by a lens, and they are using the multiple images of their U shaped pupils to gain color information.