There is an ammo can cache in my neck of the woods locked shut with a combination lock. On the ammo can are several different number combinations to try. There are about 13 total, but at least you know (hope) the right one is there.
In your case, maybe you could set up a multiple choice in the cache page or in the hint. Or, better yet, add a small piece of paper with the different choices to the first stage (with enough copies for all cachers to take one with them) to make sure people did not bypass it altogether. Something like A- 300 Green, 200 white and 50 red. B- 200 green, 250 white and 75 red, etc. You could add as many possibilities as you would like. This way the cachers might have to spend a bit of time on the cache and trying to count the marbles would help in figuring out the cache, but those who were counting averse, would still have to find the first stage and know one of the choices would yield the right answer and the right place to search.
Good luck!