Do turtles recognize colors?
The hypothesis holds true: The turtles recognized colors. Turtles are able to differentiate between colors. They are also able to see well at night.
Diamondback terrapins have diamond patterns on their hard outer shells and hence, their names. They are found in North America and can live in both seawater and freshwater. Mostly, they live in brackish swamps. They feed on small fish, fiddler crabs and mollusks. The Diamondback terrapin is an endangered species, and this is because they were hunted and killed in large numbers in the past.
Conduct the experiment in a dark room that does not have light.
Use other species of turtles or tortoises.
Also, try sealing the trays with plastic food wrap (eg. Glad wrap) and observe if there is any difference in the behaviour of the turtles.
Turtle - http://en.wikipedia.org/wiki/Turtle
Diamondback terrapin - http://en.wikipedia.org/wiki/Diamondback_terrapin
Turtles and tortoises - http://42explore.com/turtle.htm