Maybe this is how ColorMaterial is supposed to work, but I editted the geometry / vertexData of a Mesh ( heightmap-like), but when I render it, it has a very flat and solid color ( see attachment)

I tried playing with the ambient values and stuff, but to no success.

Is this a UV-thing?

and the material has a lightPicker defined with a light?

Yes, I did have that. I double checked.

I did some more searching and found that setting

autoDeriveVertexNormals = true
autoDeriveVertexTangents = true

Solves it.