Disclaimer: I'm not an expert in this field.
I define "color" as a visual perception of a spacially homogenous electromagnetic spectra. Notation: c=p(s), meaning color is a perception of a given spectra. It turns out, that these percieved colors form a three dimensional vector space, this vector space is the representation of the infinite dimension vector space of electromagnetic spectras (c_i=p(s_i), p(a * c)=a * p(c), p(c1+c2)=p(c1)+p(c2) ).
We can parametrize this vector space with three numbers. If these numbers are coefficients of a linear system in these space, then we call this parametrization a "linear colorspace". Then physical blending of different spectra results in the same blending in coefficients.
Often we don't use this kind of parametrization (most often sRGB on the web), because perceptual distance is not linear in a linear colorspace, so we can get away with 8 bits per color channel to not introduce obvious banding in the darker regions.
Blending problems occur if someone doesn't know if the three numbers representing a color accually is a member of an actual vector space or not. Nobody would attemt to use the same algorithm in HSV tö blend.