I think its happening because the eno makes bubbles when the food coloring mixes with the bubbles it makes blue blobs of stuff and it floats around the oil
When I read up on why this happened I thought I was quite close - The ingredients in Alka-Seltzer combine with water to form a gas called carbon dioxide. The oil and Alka-Seltzer do not combine in this way though. The Alka-Seltzer tablets sink through the vegetable oil until they reach the layer of colored water. There the Alka-Seltzer dissolves in the water and forms a gas called carbon dioxide. The gas is lighter than the water and oil, so it bubbles up, taking a bit of colored water with it as it moves through the oil layer. You should have seen those bubbles, looking like colorful blobs, float through the oil layer to the top of the jar. At the top the bubbles should have burst (releasing the carbon dioxide gas), and then the colorful blobs should have sunk back to the bottom (now without carbon dioxide gas). The effect should have been reminiscent of a lava lamp.
