The gamut boundary description (GBD) of multiprimary color displays (MPD) is important for color gamut mapping. Dependent primary color (DPC) is the color stimulus of a controllable color channel represented by an additive mixture of positive amounts of other primary colors. We propose a method to obtain the GBD of a MPD with one DPC. Further, the ideal color volume in CIE xyY and CIE L*a*b* color space with complete-controlling white channel is about 218.8% and 144% compared with the one of digital light processing with incomplete-controlling white channel. The white channel with complete control will not only increase brightness, but also expand color volume.