Differential phase contrast microscopy (DPC) provides high-resolution quantitative phase distribution of thin transparent samples under multi-axis asymmetric illuminations. Typically, illumination in DPC microscopic systems is designed with 2-axis half-circle amplitude patterns, which, however, reduce the temporal resolution of DPC, precluding observation of high-speed phenomenon. Efforts have been made to achieve video-rate DPC by using tri-mode illumination or adding multi-colored filter. However, the frequency responses of the PTFs has not been improved, leading to poor phase contrast and signal-to-noise ratio (SNR) for phase reconstruction. We present a video-rate isotropic quantitative phase imaging (QPI) method based on color-multiplexed differential phase contrast (DPC). In our method, the illumination source is modulated by an LCD into an annular color-multiplexed pattern matching the numerical aperture of the objective precisely to maximize the frequency response for both low and high frequencies (from 0 to 2NAobj). In addition, we propose an alternating illumination scheme to provide a perfectly circularly symmetrical phase transfer function (PTF), achieving isotropic imaging resolution and signal-to-noise ratio (SNR). A color camera records the light transmitted through the specimen, and three monochromatic intensity images at each color channel are then separated and utilized to recover the phase of the specimen. We present the derivation, implementation, simulation and experimental results demonstrating that our method accomplishes high resolution, noise-robustness and reconstruction accuracy at camera-limited frame rates.