1. An encoder for video coding, the encoder comprising: an input configured to access a video image comprising a luminance channel, a first chroma channel, and a second chroma channel; and a processor configured to: generate a compressed bitstream of a first image region in the video image, comprising: compressing the luminance channel of the first image region using a first luminance quantization parameter; compressing the first chroma channel of the first image region using a first chrominance quantization parameter; and compressing the second chroma channel of the first image region; generate a compressed bitstream of a second image region in the video image, comprising: compressing the luminance channel of the second image region using a second luminance quantization parameter, wherein the first luminance quantization parameter is different from the second luminance quantization parameter; compressing the first chroma channel of the second image region using a second chrominance quantization parameter; and compressing the second chroma channel of the second image region; and signal the compressed bitstream of the first image region and the second image region, the first luminance quantization parameter, the second luminance quantization parameter, and a first quantization bias to a decoder, wherein the first quantization bias indicates a difference between the first chrominance quantization parameter and the first luminance quantization parameter, and wherein the first chrominance quantization parameter is less than or equal to a predetermined maximum value.