Good topic.
In my opinion it's got to be a combination of colour, size, shape and frequency of vibration (which varies with density of plastic for soft lures) that turns fish on.
I have a personal theory that counter-shading light and dark on lures works because it's easier for fish to see e.g. alternating flashes of light colour is more eye-catching than a constant light silhouette.
As for what they see... once the lure is deep enough that all colour is gone, I suppose it's the silhouette and how much contrast the silhouette offers that they pick up on e.g. whether black will stand out more at depth?