Rajesh - In your screenshot, you have the view focussed on the Selected Data subset, which is empty if no rows are selected in other views. Because the subset in the view is empty (no rows selected), the colour key option is greyed out. What happens if you select some rows in another view such that the target view is populated with non-null rows?

You can disable the colour key from the main toolbar instead

Thanks for the response Daniel. Now it works fine