Looks like the color codes are applied at Checkout, not when you put it in the cart. I tried to put several one-year rooks in the cart, at different times, having different colors, but the cart merges them together into the current color.
So if someone wants to buy multiple color codes (which is only interesting if you try to get them all, otherwise you would always use the highest), you need to checkout for each color separately, and produce multiple orders (I'm pretty sure you can still pay them all in one go).

Also, this means that if you are trying to catch the Easter-Egg bonus, you can already prepare your cart, and when it appears - even for 15 seconds only - you can just quickly check-out and get it. :-)