For example, it is easier to tell the difference between a hard or soft bottom. The soft, muddy, or perhaps, weedy bottom returns a weaker sonar return than a hard bottom such as sand or rocks. When this signal difference is in color, differences are apparent.
Similarly, weak signals such as fish in weeds (e.g., milfoil) or fish mixed in with thin tree limbs will stand out with a properly adjusted unit as do fish lying close to the bottom. Programmer/engineers assign the colors based on signal return strength, similar to radar returns on television’s Weather Channel. Generally, weak returns or echoes are assigned cold colors (e.g., blue). Medium colors show with warm colors (e.g., green, yellow). Finally, the warmest or hottest colors show the most intense returns (e.g., red, orange).
^^^This was copied an pasted from a tutorial site.
Basically the harder the bottom compostition the more red the color will be. Your true solid rock bottom will be red - most will be a mixture. There are some other colors I have found that are specific to certain things - like a pretty good shell bed on Ky Lake, will show up almost as a brownish color over a redish bottom.
The best way to tell the bottom compostition (or at least for me) is to toss out a carolina rig and feel the bottom.
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