Why does spectral class and U-V colour correlate slightly differently for main sequence, giant and supergiant stars?

In this diagram the $x$-axis is spectral type for stars and the $y$-axis is B-V colour.

Taking Series 1 as the Main Sequence, Series 2 as Giants and Series 3 as Super-giants. Why the spectral class for all these stars differ from the other ones?

If we were to say differences, for instance, how would one express that information?

• What is measured on the vertical axis? Commented Nov 19, 2015 at 17:52
• Hi there.The vertical axis has the star colour B - V. Commented Nov 20, 2015 at 0:46
• This question doesn't strike me as clear, but I think what's being asked is "Why do stars of the same spectral type have different colours at different brightnesses?" In his case, for example, a K5 dwarf has B-V=0.8ish, vs a K5 giant with B-V=1.1ish. (Based on the plot above. No idea where it comes from.) Commented Nov 20, 2015 at 7:56
• This is clearly not the Hertzsprung Russell diagram en.wikipedia.org/wiki/Hertzsprung%E2%80%93Russell_diagram Commented Nov 20, 2015 at 23:49
• Okay, the vertical axis is the B-V colour index, as explained here en.wikipedia.org/wiki/Color_index Commented Nov 20, 2015 at 23:54