I’d look into ray tracing. For free form shapes, lets say you have a plane. in the material of the mesh, you’d get the hit location and change the vertex colour. You can then use that vertex colour as an apha for a lerp between 2 colours. 
I’d look into ray tracing. For free form shapes, lets say you have a plane. in the material of the mesh, you’d get the hit location and change the vertex colour. You can then use that vertex colour as an apha for a lerp between 2 colours.