In the context of game engines with graphical user interfaces, shader programs for GPUs (graphics processing units) are an asset for game development that is often used by artists and game developers without knowledge of shader programming. Thus, it is important that non-programmers are enabled to explore and exploit the full potential of shader programs. To this end, we develop principles and guidelines for the design of usercentered graphical interfaces for shaders. With the help of several examples, we show how the requirements of a user-centered interface design influence the choice of widgets as well as the choice of the underlying shader parameters.