there are 6 or 7 of them and they are used for different things, most of the info I got from the cage manager when I asked about them.

black - was used as a bonus for converting Canadian dollars (C$500 got the daily exchange rate plus one NCV25). This promotion is no longer done
blue - used with a coupon for new members sign up bonus
yellow (with blue/green insert) - used for locals who get promo dollars in lieu of rooms
lavender - used during promotions like the triple crown promo they had recently as a prize
green - told this was a giveaway from the owners who meet people on planes and other public places as an incentive to come down to GG
yellow (cant remember the inserts) - ??

There are the NCV5 equivs of most of them as well.

I've got the black, yellow, blue.