I wonder how much of the shade cloth they can see. I mean, from the outside, how much of a distinction can they make, or what landmarks can they see to navigate back in. I would think ti wouldn't matter too much what colour you made the entrance, just so long as you distinguish it from the surrounding expanse of cloth. Although I seem to recall they don't see red all that well but I reckon white or yellow would work well. Maybe you could also erect a landmark above your hives so they have something else to orient to.
You stated in an earlier post that you were thinking of rotating hives through the term of the contract. What is your reasoning for this? I would have thought there will be an initial huge learning curve for each new hive that gets placed on location but the newly emerging bees from an in-situ hive will have less of a learning curve. As long as your population numbers stay fair I would be inclined to leave the same hives there.