Edit - no purple in my French tree because I finished the tree, but purple is in my German tree. Purple means you haven't reached level one yet. The lessons will change colors as you progress through the levels.
Yes, I heard that 1 is blue, 2 is green, 3 is red, 4 is orange and 5 is gold, but I don't have the colors yet. Only some people have them so far, lucky you! This is the first time that I heard about purple, so thank you for the information!