You could use the boolean instance variable to turn on 1 green ball only.
Then you can do the event with the green ball which boolean is true.
Thank you for your reply. But sorry, I have tried with instance variables but couldn't get any decent comparison to make that will particularly differentiate only 1 green Ball at a time. Trying with instance variable either makes the Redball stop rotating or all the green Ball stick to the Redball. I know I must be making some silly mistake as I am new to Construct & it would be really helpful if you can give a clear explanation on how can I use the instance variable properly. :)