I've heard several answers to thus question, and still don't know the real answer..... Does Dumb Blonde have to be used on blonde hair only?

Not necessarily. It's basically formulated for lightened hair. Processing color out of hair can often leave it dry or even damaged. It also leaves hair prone to absorbing minerals leaving that yellow brassiness color you commonly see. This product is a decent affordable balance of moisture while also removing brassiness. It won't lighten your hair but may not be the right balance of moisture if you have a different hair type. I would suggest talking to a professional stylist the next time you get your hair done and ask what they recommend. I noticed that during an appointment my stylist had used this product so I asked and she actually recommended several things including a leave in pair of products. I've never had breakage in my blonde for years following my stylist's recommendations! Everything mentioned goes well with the exact "magic" she puts into my hair during processing. My hair is naturally a dark blonde bordering on light brown but I have it processed to a platinum blonde which I then shampoo in a pink washe. That means my hair takes an absolute beating and has for a few years now. While this product is excellent, it does not fully remove brassiness (when my hair is blonde) and it is lacking a little added moisture so I do a mask to help and as mentioned pair two leave ins. So, after all of that would I say use this on any other color? Sure, if you are on a budget. If not look into a specific formula for YOUR hair. Function of Beauty (website is one word) makes custom blends for $50 and you get 2 x 16 oz. This product at $21 for 2 x 25oz (Aug 2020) is more affordable to me. Good luck with your hair!!!

I was wondering does it lightens the color of your hair

Not unless you have colored hair and even then it is very subtle over time like any shampoo. Though this is formulated for "blondes" and removes brassiness, that will not remove natural color or lighten existing blonde. If you are like me and use fun colors over top of blonde then you will see color go down the drain. I recommend a washe if that is the case which is a shampoo plus a color which is not permanent. Most all come out with Carol's Shimmering Lights shampoo in a short time. Don't take my word though (: read the labels and ask your stylist what is best for your exact hair.

What "damage number" would this be? I've used it before and love it, but how does it compare to the Resurrection?

Resurrection is level 3. Resurrection is much stronger and works on more damaged hair. This would be down maybe a level 2, but it is really just a daily use for color damaged hair. It's great but you might pair it with Resurrection still.