Hey there! I've done a few keratin treatments within the last couple of years and I've had great results in all honesty.
The first time I did it, wasn't so great. I have 3c curls (4a in some areas), and it didn't really do much. It gave me a combination of 3b/3c curls but it helped out tons in terms of frizziness. I could leave the house with my hair still a bit damp and without any product in it and I wouldn't have to worry about it frizzing up everywhere. Of course, in humid weather, it would get a little puffy, but it never got so bad that I actually had to pull it into a pony tail.
The second time I had good results. My curls were probably somewhere around 3a/3b and I liked it because I didn't have to worry about too much poofiness and I still had some sort of volume to my hair.
Now, the last time I had a keratin treatment done, I had the most obvious results. I'm not sure if it was because I'd done it before (even though I waited about 13-14 months since the second time before I redid it), but for whatever reason, my hair turned out a lot smoother! All I did was apply a tiny bit (about dime-sized) of serum to my hair and leave it to air dry and it would dry completely straight. At first I loved this, but then I got annoyed because I missed having curls. It took a good month before it stopped being so straight and started getting wavy instead. However, now (about 3.5 months since my last keratin treatment), my hair is a mix of 2b/2c/3a curls (depending on the area), however my roots are obviously different because my hair is growing out.
As for color treated hair, keratin really does a number on the hair dye, which is why I only dye my hair after getting the keratin treatment done. You'll find that it fades the colors away pretty quickly in the process of the treatment, which was a bit of a bummer because I had to redo it.
As for damage, keratin has only helped my hair! I haven't noticed anything wrong with my hair and after getting the treatments done, I've almost completely stopped using hair straighteners (I used to straighten my hair almost every day). So my hair is definitely getting stronger and healthier and it's pretty evident by taking a look at my roots.
The only downfall for me was that my hair grew in a lot faster than the keratin faded away, so it's sometimes awkward having to deal with much curlier roots. I hope this is enough information for you, but you're welcome to ask me any questions if you have them.
Also, do remember that everyone's hair reacts differently to a keratin treatment. I've seen some people who have had horrible results and I've seen others with great results, it ultimately depends on your hair and how it takes to the treatment. Your best bet would be to go to a trusted stylist who has done this before and knows what he or she is doing. Have a talk with them first before you go through with the treatment and see what they think.