So you just went out and bought yourself a fancy new color unit. Get it all set up, head out on to the lake and start graphing, when you realize you're not really sure what all those colors mean. You scan the message boards and hear things like "it's just like color weather radar" or "yellow is hard bottom stuff". Well, kinda', but not really (LOL). Simply put, when the transucer receives a return signal, that return signal has a certain strength associated with it (expressed numerically in the little computer chips brain). That strength level then gets assigned a certain color band to represent it. So the stronger the return signal the higher up the color band that signal will be expressed. Take a look at this next graphic showing the Lowrance default signal strength color band for a better understanding.
Where things get a little tricky is with interpretation, especially when taking into account user settings. You could adjust things so that you never see a yellow pixel on your screen if you wanted, where the stronger return signals were more magentas and reds. Doesn't mean anything as long as you understand the basic principles behind color. Some color will always represent the stronger return signals and other colors will always represent the weaker return signals. The fine tuning comes in optimizing settings (colorline and sensitivity) to get the maximum (color) distinction possible across the widest range (strengths) of signal returns. When you've accomplished that, you've completed the first step in better understanding what your unit is trying to show you.