Blue is Ishihara Kaori, pink is Ogura Yui. In their first two singles they stuck to that general color scheme, but their 3rd and 4th singles their main outfits completely matched each other (both white and both yellow), which makes it not quite as easy to tell them apart if you're having problems. But yes, as a general rule, Kaori will be wearing blue/purple-ish, and Yui will be wearing pink/red-ish. Kaori is a little taller and has more stark features, where Yui's face is rounder and elfish. Also, when singing, Kaori will be the one with a deeper voice, and Yui will be the one with the unimaginably high-pitched anime voice.