Sap2000 and I guess all finite element software use colors to show ranges
In your case, the colors will show you how deflection varies in your shell elements.
Click on the shell element and you will see values of deflections as you move the mouse.
The chart at the bottom shows you the range. For example, if you are using mm for units and if red is shown between number 1.75 and 2.25, that means any area on the shell that has red color in it, will have deflection between 1.75 and 2.25 mm.
Isnt that useful?
Very useful when you are dealing with shells. Because there are usually too many shells and as an engineer you want to concentrate on a few of them.