In traditional 2D hard bottom is red/deep red
It depends on which color pallet is chosen. It could be black, red, yellow, or green.
To determine soft from hard bottom regardless of the color pallet, the thicker the bottom return you have, the harder the bottom.
Hard bottoms return the strongest echos.
With Imaging, the brighter/light bottom areas are harder than the darker softer bottom returns.
If the Green/Red pallet is used, RED will indicate hard bottom areas. That pallet was designed to easily show hard to soft bottom transition areas.
Additionally with Imaging, surfaces that are perpendicular to the sonar pulse will have brighter/lighter returns than surfaces angled away from the sonar pulse.
The creek channel drop opposite the sonar pulse is displayed brighter: