Kelvin is based on personal preference. It wouldn't matter to clams weither it's under a 20,000k bulb or a 6,500k bulb. If the clam is healthy and it's kept in the proper conditions with the proper intensity level (meaning that the bulbs have a high P.A.R. rating), it should grow just the same under different kelvin ratings. It all boils down to intensity, not coloration
By the way- I couldn't help but showing off my T. Crocea (taken with flash. No editing was done on the photo).
Take Care,
Graham