For example, a star which is magnitude 1 in the V filter, and magnitude 2 in the B filter, is fainter in the blue and so is a redder star. It has a B-V of 2-1=+1. So the larger the B-V value is, the more red it is, and the more negative it is, the bluer it is. Note that traditionally in astronomy the bluer of whatever two filters are involved comes first when we make a colour, so we speak of B-V colour but never V-B, and B-R colour but never R-B. This is just a standard which ensures that positive colours always indicate redder objects and negative colours always indicate bluer obje