added the fact that the color is band1 - band2, in analogy to g - r color
added the fact that the color is band1 - band2, in analogy to g - r color

sncosmo/sncosmo,sncosmo/sncosmo,dannygoldstein/sncosmo,dannygoldstein/sncosmo,kbarbary/sncosmo,kbarbary/sncosmo,kbarbary/sncosmo,dannygoldstein/sncosmo,sncosmo/sncosmo