somehow it seems that distinct 'colors' should not be hyphenated to be used as lables for naming gluons, because the eigen-value for, say, green-ness communicates the quality of that color-charge which we call green, and which is, in spirit, orthogonal to the other unique color-charge qualities. if then, say, anti-blue is added to the mix, it brings its own distinct quality. with two qualities to identify, it seems that we would need to use two eigen-values, but by convention, we use only one. we do this by hyphenating the two color-charge qualities and considering their combination to be a label for this gluon. does that make sense?