Well, being a fisherwoman I have actually caught squid to use as bait. I have noticed that when the squid get excited the ink that is under/in their skin begins to surge, almost making it look like they are boiling. They can go from a really pale fleshy color with some brownish red spots, to almost purple. Perhaps it is a form of camouflage or intimidation as the movement of pigment can be very rapid. They can and do shoot ink when they are upset but they can also swim very fast. They create almost "jet power" by forcing the water out through their mouth.
Rapid movement and ink projection.
Many use rapid escape and camouflage and also have an ink sac. The rest become calamari.
They project ink that takes the form of a squid and they make a quick get away.
I think they also can lose a tentacle and regenerate it as well as the inkie getaway.