A design patent protects the aesthetic outer design of industrial products regarding colour and composition. You can either protect two-dimensional designs (samples) or three-dimensional designs (models).
An efficient design patent has to be new, i.e. design features are not yet known in domestic circles of experts. It has to show a characteristic, i.e. the design exceeds common knowledge and is given distinction by an individual creative achievement of the author; additionally it has to be commercially useable.
Please note that a design patent, similar to a utility patent, is an unchecked property right. The DPMA (German Patent and Trademark Office) only checks formal aspects.