Hey there Don. I don't think the 585c is as much of an upgrade to the 565 as it is another model (the color model in this case) in the 500 series sonar units. If you look at all the color units on Humminbird's website you will find that the price increases with an increase in resolution. I can't say for a fact but I'm sure it costs more to produce per color pixel than greyscale and the 585c is the bottom end affordable unit.
I have recently put a 585c on the front of my boat and I would say "yes" when the sensitivities are set right the color makes a difference. Before, with my Matrix 17 (320x320) I could see good detail and fish around the condos. Now with the 585c (240x320) I get the same detail and fish around the condo but I can also see fish inside the condo. The cover is usually blue or green (less dense) and the fish are orange or yellow (more dense).