I try to use the Color Correction effect on one of my videos. I do the following steps as i read in help:

-Activate "Add".
-Place the mouse over the preview monitor. The mouse pointer turns into a pipette.
-Click with the pipette tool on the color in the program monitor that you would like to assign to the mask. Unwanted colors can be removed from the selection again by selecting "Remove" and clicking the corresponding color.

At this exact point as i try to choose the area spots for color correction all the video gets black. What i am doing wrong?