My question is if it is possible to select a certain part of a video and let it keeps it colors whilst there rest is black and white? I've tried messing around a bit with colors but maybe you guys know perhaps a better way to this or if this is possible at all.

Depending on what you want to do you either use Alpha Shapes, Shape Alpha, or Rotoscoping.
Alpha Shapes is limited to rectangle, ellipse, triangle or diamond. Shape Alpha takes another image as the mask. And Rotoscoping allows you to create a mask around objects and also keyframe it.
In all cases you need the two clips to be on top of each other in different tracks in the timeline. The top one needs all the effects, e.g. greyscale and then of course the masking effects. Make sure when rotoscoping that you select Write on clear for the Alpha Operation.
Hope this helps.