Composite map with falloff -light shadow render error

[stevesideas]

Im using a composite map in the diffuse slot on some tree models. The 2nd layer is the tree bark and the top layer is a moss texture. So im trying to get the moss to be on the darker side of the tree. Ive dropped a falloff map with a light/shadow selection into the mask of the top layer. The render comes out blue...is there a reason for this ? or another way of doing it ?