I did some testing with the trial of NeoHD, but now i am stuck with some clips that have been colour tweeked.
How can i remove this. These clips which are on my disk are the original ones.

Not stuck. Many options. Easiest is to turn off the Active Metadata features in the Status Viewer, "C" icon in the systray. Or delete the database (if you still have FirstLight.) There are other options, those are the easiest to start with.

Is it ok, that systray ikcon isn't there all the time? It shows from time to time, but it dissapears after an update check...

It is hasn't disappeared, Vista and Win7 hide infrequently use icons, look in that collection.

Not stuck. Many options. Easiest is to turn off the Active Metadata features in the Status Viewer, "C" icon in the systray. Or delete the database (if you still have FirstLight.) There are other options, those are the easiest to start with.
I turned it off in the AM but should it not revert to original when deinstalling NeoHD?

No it shouldn't. Imagine creating images in photoshop, then uninstalling PS and having all you pictures automatically deleted. We consider you color corrections to be user generated output, not to be deleted on an un-install (or even accidentally, so a user option also makes no sense.)

Point taken.