How does the "colour" of an object indicate how much heat they will absorb/reflect ? 

Different colours absorb different amount of heat. Black colour absorb more heat then that of white colour so we use black colour in solar cooker and white colour's t-shirts are used while playing in sun light.The observable colour of an object has to do with the light wavelengths that are reflected from that object. This reflection, in turn, depends the atomic and molecular properties of the object, its surface structure and the angles at which the light hits it and at which someone observes it, among other things. An apple is red because when white light, which is made up of all visible wavelengths mixed together, hits it, its atomic innards reflect all the red wavelengths more than the other colours and bounce them at our eyes