If the sticker you have on your bike is the correct sticker provided by the DMV for your bike, the rangers can't do anything about it. If you have a green sticker, good for you! That means you can ride almost anywhere, almost anytime. You DON'T want a red sticker because there are restrictions as to where and when you can ride a red sticker bike.
Get your registration paperwork together and take it with you next time. As far as I know, all you need at Hungry Valley/Gorman is a properly registered/stickered bike (green sticker can ride most of the year, red can ride for some of the year) and a legal spark arrestor. The rangers can't deny you access if you can prove that the sticker on your bike was issued by the DMV; they are there to enforce the law, and the law says green stickers can ride there.
Be nice about it, but don't back down. If they give you a hard time ask for a supervisor and make sure you get a badge number. There is a definite problem with California's Green/Red sticker program, but the rangers are supposed to enforce that you have a sticker and it is the correct color for the riding season in question. If the DMV gives you the wrong sticker (or the BETTER sticker), it's not up to the rangers to prevent you from entering.
Consider yourself lucky, my WR450 probably pollutes less than your CR and I get a red sticker :-(