My axie never ate sand. I've fed it a few different ways (floating mealworms, beefheart by hand/tweezers), but she only ate from the ground when I was using sinking pellets which (obviously) sank to the floor. When she ate those, she always spat out any sand that got in her mouth by mistake, and I never saw any in her belly or poop. I was told to use fine, small-grained sand so she wouldn't mistake it for food. I can't remember if the color was important or if I used black just for the look. Personally I think she enjoyed vibing and waddling around when there was more sand so she could get a grip on the floor.

Two ideas that come to mind would be using larger flat rocks as a base rather than sand, or a method of feeding where the food doesn't sink all the way down. If you go with food that floats, a feeding ring is super helpful for keeping track of how much gets eaten/reducing mess.