If I am not mistaken, he sometimes dual wields a blue and a green.

Lots of characters wield different colored lightsabers at different points in their career. During the Republic comics before Attack of the Clones you'll see a full rainbow. Depa Billaba, Adi Gallia and Even Piell even used red lightsabers. And don't forget, in Revenge of the Sith itself, Agen Kolar uses a blue lightsaber after using green in all previous appearances.