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I had an ounce of brilliance for a moment, and tried an experiment.

I stuck a piece of gravel, the color of their food that they currently eat, inside of the container. They almost ate it, until i came over, and they dropped it.

I took it out, and stuck in a piece the color of what they arent eating (veggies). They didnt even sniff it.

In conclusion, if you plan to use gravel, use the color of the opposite of what they are eating.

From what I've read, almost everyone who has kept RES with gravel/small pebbles and later removed them would later discover that their RES did eat some of them. Using gravel is a risk - so is putting anything in your tank that is not intended to be eaten.

Same warning with other animals in your tank - RES are omnivores and will instinctively go for smaller prey.

My two girls that I rescued have been out of a "gravel tank" for about 3 years (that is if the former owner didn't lie about it) and I am still finding pieces of gravel at the bottom of the pond. Scary eh!

It's best not to use gravel at all. Why tempt them with it even if it is a color that's different from their food??