When you mix big rocks with little rocks, and let it sit long enough, the big rocks end up on top of the little rocks. As I understand it, the reason is because the little rocks can fit easily in the gaps under/around the big rocks, so tiny vibrations cause them to migrate downward. Back in the good old days, when I was a kid, my family always had a big vegetable garden, but no matter how many bushels of rocks we raked off the soil, within a month the top of the soil was covered with rocks. I thought it was magic then.
All pool filter sands are around 1-3 mm diameter particles, as far as I know. The zeolite sand I tried, "Zeosand" was about that. So, if the color quartz particles are smaller they would migrate down into the zeolite sand, leaving the bigger chunks of zeolite sand on top. If you read up on Eco Complete, they use that as a selling point - that substrate automatically adjusts itself so the fine particles are on the bottom and the bigger ones are on top.