Check out a sample game of one of the original Pyramid games, played with the game's creator Kory Heath! Back in the Lab we're working on a new version...
You can make the rules as easy or as complicated as you wish, but until you have played many games, it can be deceptively easy to make a rule too complicated. I knew a couple of guys who played Zendo at lunch at work, and one of their rules was that the total number of pips (the indents on the side of each pyramid) in each set had to be a prime number. For a while, I'd recommend easier rules like "must have at least one blue piece" or "Must have a small piece pointing at a large piece". Then work up from there are you are comfortable doing so.
Following up on rustedbeetle's comment - you absolutely can make the rules as complicated or simple as you'd like. Be on the lookout for something new involving Zendo in the next year or so as well!
Was it a green piece or at least one green piece? Not sure how you would disambiguate if it's the latter, given that a single green piece would fit both rules.
If the rule was "at least one green" and Andy guessed "exactly one green", Kory could build a koan with two green pieces and say "this fits my rule, but not yours." If the rule was "exactly one green" and Andy guessed "at least one green", Kory could build a koan with two green pieces and say "this fits your rule, but not mine."
