Electrically Controlled Birefringence (ECB) cells are often used to fabricate switchable color filters. The color filter we consider consists of 3 polarizers and 2 birefringent modules. Each module consists of a combination of ECB cells and retardation foils. When a maximum response time is taken into account, one can determine a minimum voltage drop that has to be across the ECB cells. This results in a smaller interval in which the retardation of the cell can vary. Since different colors are achieved by varying the retardations of the two modules, it will be more difficult to obtain a saturated red, green and blue color. In this paper we describe a graphical method to design a fast switching color filter with saturated colors. The method can be used for different kinds of liquid crystal cells.