I think how much light depends on the species. Color of the clam is not what determine the amount of light the clam need but the species of clam and the strain of the zooxanthellae the clam carry determine how much light the clam need to do well.The population of zooxanthellae in a specific coral or clam is a heterogeneous population. Each coral or clam host a numbers of zooxanthellae strains which may differ in the optimal amount of light and wavelength they need to do well. That is how the host animal adapt and continue to do well at various Kelvin and light intensity.
The pigments of the host are protein in the tissue that are likley use for light protection. Low light condition tend to cause decrease these protein result the host loose the coloration and turn golden brown which is the color of the zooxanthellae. When the host bleached, meaning loosing much of the zooxanthellae, they loos that golden brown color result in pastel color of the pigment. Also as the host bleach, it start to loose energy, which intern will cause it to loose color pigments also. So early in the bleaching process, the pigment protein still there and the host look like pastel florescent color, later in the bleaching process, the animal become essentially lucent white (or white white due to the white skeleton)
