Here's one of Steve's Time-Lapse videos showing how running water can generate gorgeous color patterns.
Steve Gough posted a comment, but it was eaten alive by Google:
This is super interesting to us at LRRD! We ran a lot of specific gravity tests as we developed the media; and found very close clustering, but more at about 1.60. I'm not too concerned about a 0.05 difference between colors, but if it got bigger I would be! Melamine has very low water absorption (for some plastics it can be 10% plus), but it's possible that's part of the problem; you might try soaking all the samples for a few days before testing. Thanks!