I'd honestly think the flower would darken up under any sand, either dark or light. I've had mine over both bare bottom and light sand, and they're definitely darker with even the light sand than the bare bottom.
that older flower I had was 18"+ but had a very short tail. i have been very lucky with flowers. have owned 5 and had good luck with all of them so far, fingers crossed but this one eats the most different types of foods and all. They are my favorite rays beyond black ones for sure