Terrain detail meshes with strange colors

I am trying, without success, to paint detail meshes onto the terrain but they come out with very strange colors. You can see below a dock leaf model. The lighter one is the prefab set with the nature soft occlusion leaves shader...this one looks great. Once added as a detail mesh and set as grass, however, it appears tinted deep green with some parts of the mesh entirely black. I've tried changing the dry and healthy colors but with little change.

I am guessing this is something to do with vertex colors but almost nothing is given in the documentation specifically about it (apart from using the alpha bit for blend). This is rather frustrating...I've used UE4 and the foliage painter there works flawlessly out for the same models.

So are there any specific requirements for detail meshes beyond using a single material? Do they have to have vertex colors on import and if so, what values are recommended? I've seen previews showing perfect looking detail meshes in some tutorials, but no mention of how they were achieved.

Many thanks