The formula says:
Y = 0.299 * R + 0.587 * G + 0.114 * B; U = -0.14713 * R - 0.28886 * G + 0.436 * B; V = 0.615 * R - 0.51499 * G - 0.10001 * B;
Y, U and V are all allowed to be negative when represented by decimals, according to the YUV color plane.