Spaces such as YUV and YCbCr are not "color spaces" really. They are simply an encoding of an RGB space, and any definition of them will include the values of the R, G, and B primaries and the fact that they are YUV or YCbCr encoded. There is no such thing as a "Y primary" and certainly no such thing as a "Cb" primary (since there .5 is black!).
Claiming these are color spaces is equivalent to claiming that jpeg is a color space, rather than a compression algorithm.