Hi, I'm trying to make my movie in black and white but i have a greenscreen, so then the greenscreen turns gray, so its not green anymore so it doesn't work. Any way to make the actual greenscreen stay green?

I would edit in color using the green screen as needed. When you are done I'm not sure if it will let you apply filters to goto black and white without messing up the green screen stuff so if it doesn't, export the full resolution color movie and then start a new project and import your final edited full resolution movie and then convert to black and white.

Kind of a roundabout way but I don't see any reason why that will not work for you.

You have to export the movie. Reimport it then add the black and white effect as stated earlier.