Herk Man,
THIS IS A SERIOUS RESPONSE.
As it happens, I know a fair amount about manufacture of foam laminated products, both from manufacturing experience, and from extensive conversations with my ex-father-in-law (a double PhD) who devised the foam formulations and process that helped found the lamination industry.
It would not be unusual for different colors to be sourced from different bonders. So, the answer might be as simple as that, since no 2 machines (even in the same plant) produce absolutely equal products. Also, there might be minute differences in the actual thickness of the foam.
But, assuming that he same machine if used for all colors and black, the answer becomes a bit more complicated. I doubt that the actual physical weight of the pigment would give the disparity you recorded. However, the addition of a different color pigment to the yarn, assuming it was vat dyed, could cause a bit more stiffness in the surface fabric, thus causing differences in compressibility.
I am sure that you know that to produce bonded fabric, a 3 layer sandwich is run at very high speed over heated rollers. My thought is that, if no adjustment is made to the temperature for color, that the black fabric would absorb more of the heat causing a slightly greater melting of the bonding foam. This would "collapse" a greater number of the foam's pockets leading to a reduction in buoyancy. OR NOT.
Interesting problem.
