Here on Alla's in the trade skills area, Green and Gray are referring to skill levels. The skill spectrum goes:
Red - Cannot be made
Orange - Guarenteed skill increase
Yellow - Often gives skill increase
Green - Rarely gives skill increase
Gray - Never gives skill increase
Why only the green and gray values are listed, I don't know. When you're working on skills, you generally want to make yellow+ (preferably orange) stuff.