You still own your work. They do not. If you ever decide to publish your work later, you still own the rights to your work.
The courts have ruled that while Turnitin can retain copies of your work and use it, they are not profiting from it, nor are they in violation for keeping your work. They are using it in a modified matter. The courts believe that they have, in effect, used your work in a way like a derivation of the original; they have taken it and made it into data, not prose. Turnitin uses your work, not as an essay, but as a pattern of words which it searches for matches. Turnitin doesn't care what you wrote, it just uses the pattern of words and looks for matches (examples of plagiarism).
Turnitin uses color codes to quickly identify if one work is too similar to another.
Should you worry about the colors, if you can see them when an item is turned in? Maybe. It depends.
If an entire class is required to outline the same Biblical text, will everyone's outline be red (total plagiarism)? Yes. Did anyone plagiarize? Don't know. The professor had better pay attention to, not just the words or color, but context and format. This gets even more tricky if the professor has used the same passage multiple years.
If there is only one phrase that is a match, should I be worried? Maybe. All it takes is one phrase, even just two words, for something to be plagiarism. So, even if just two words are a match, you just need to be sure that they aren't words that come from someone else; that they are original words by you (or common phrase or common knowledge). When in doubt, cite.