My group is "Keep the spot alive".

Is it a warning or ...?

No, it's just one of the weird member group names that djp decided to go with.

why dont i have it?

Because you don't have enough posts.

I also notice the clolor of my username is different.

Feel like unwanted here

I thought it was because he has donated or something :S

It just means that you are above the other members due to your higher amount of activity. It isn't a bad thing, it's a good thing. You stand out from the other members.

So, you have less than 90 posts you are a member, 90 posts and you are "keep the spot alive" and then above 90 posts is member again?

I have absolutely no idea how it works, I just know it's another member group and I'm only assuming it's determined by post count. If you absolutely have to know, ask djp.

It's determined by donations.

You donate, you get the special group.