Autoencoder doesn't learn to reduce dimesions
What kind of color are you trying to learn? If it's a random color, then yep, tough luck, you can't reduce the dimensionality of a 3D Euclidian manifold to less than 3D.

To be more precise, this is the case for an infinite manifold, for non-infinite one, you could get a 1D curve that goes through a 3D cube (in philosophy, it would be close to a Kohonen map, despite having just one node instead of a set of discrete nodes), but you would need a very big set of intermediate layers to make it happen.