Some of the buttons in the Collect Data screen will become gray under various conditions if the related Event Assistant feature is turned on. When the button is gray it is still active. The color is just a guide telling you that the event you want next is likely not the gray colored event. You can ignore this and select that event in spite of the color change (if for example you are adding events out of order).
This gray state is possible for offensive rebound, defensive rebound, assist, and block event buttons.