Furuno GP-3500F-D Operator's Manual page 219

If two fish schools appear with the same color at different depths, the one in
deeper water is denser because the ultrasonic wave attenuates as it propagates
and the fish school in deep water tends to be displayed in a weaker color.
Fish Echo
Fish schools at bottom
Because bottom echoes are stronger than fish school echoes, it is possible to
distinguish them by echo color. When the gain is properly set, bottom echoes are
displayed in reddish-brown or red, and fish school echoes in weaker colors.
Other
Plankton
A plankton lager, a likely place to find fish, is displayed in green or blue dots.
Less Reddish
(Sparse echo)
Diference in
Reddish
signal strength
(Dense echo)
Fish school density
Bottom
fish school
Plankton
Fish School
Plankton
11. VIDEO SOUNDER OPERATION
Strong
Echo
Weak Echo
11-43