Color error diffusion can be classified into two types, namely: vector error diffusion and scalar error diffusion, according to the underlying quantization methods. Compared to scalar error diffusion, vector error diffusion is typically superior in overall image quality. However, it requires significantly more computation. In addition, it may occasionally introduce boundary artifacts. A color band, at a width of a few pixels to tens of pixels, may appear along the edges. In this paper, we propose two improvements in quantization for color error diffusion. First, we introduce an algorithm, which we call semivector quantization, to reduce the computational complexity of vector error diffusion. Second, we present a simple yet effective method for reducing boundary artifacts.