Dave .... is that an actual recording being played back, or is that simulator mode ??
I'm thinking simulator mode, which is likely to give you the best images because they're probably computer generated & not actual recordings in the simulator program. With that in mind, I'd say the green is bottom cover (brush, weeds, etc.) and the yellow indicates bottom hardness. Normally speaking, the yellow indicates a soft object & red indicates a hard object. With that in mind, I'd say the dark blue is just no signal return being detected.
Now, I could be totally wrong ... but if that depth finder is supposed to be in any way similar in its coloration, like most other color 2d depth finders, then that would be my best guess. I have no experience with color 2d units, as I went straight from b/w 2d units to color DI units .... and even though my latest unit (Hook 7) has DI & 2d capabilities, I always run it on DI.