I have seen people asking for various types of colors and symbols related on sketches and features on various SolidWorks forums, so thought let’s check what are these things are and what the mean.
Sketch Colors:
Blue: Indicates an Under defined Sketch (no dimensions or relations or you need to define/add more dimensions or relations) 
Black: Indicates a Fully defined Sketch (you need to use relations and dimensions to fully define a sketch)
Red: Indicates an Over defined Sketch (you have added more than required dimensions or relations)
Golden Brown: Indicates a Dangling Sketch (some reference has been missing)
Cyan: Indicates a Selected Sketch
These are the mainly used/seen colors in a normal day-to-day working. Please note that these may vary depending on the color setting on your machine under Tools > Options > System Options > Color
Sketch Symbols:
Under Defined sketch : No dimensions or relations or you need to define/add more dimensions or relations
Fully defined Sketch : The sketch has enough dimensions or relations and you need not to add more dimensions or relations
Over defined Sketch : You have added more than required dimensions or relations
Dangling Sketch : Some reference has been missing
Shared Sketch : The sketch has been used by more than one feature
Multi Contour Sketch : The sketch has more than one contour.
I hope this post is helpful. I’ll keep on adding more information on colors and symbols about different entities in SolidWorks
