U-B means luminosity of Ultraviolet minus luminosity of blue
B-V means luminosity of blue minus lumionsity of Visible
If so, reading the diagram for all main types of galaxies:
I can get to the conclusion that Elipticals (B-V > 0.8) are the bluest and Irregulars are the last ones of it comes to being blue (B-V = 0.4).
It kind of contradicts the fact that Elipticals are full of old red giants, that should be RED, not BLUE, doesn't it?