How do I "activate" the dropdownlist? For instance, I have a dropdownlist name file, under file --> background color. When I select this background color, I want the change the background color from black to white. How do I go about doing that?

I know for a controlP5 button, for example a READ button, all I have to do is to create 

void READ()

{

      background(255);

}

But this does not work for a dropdownlist. I have checked and it was mentioned that dropdownlist has sth to do with the controllerevent but I still did not manage to comprehend it after playing with it for a long while. 

Thank you in advanced :D

hanny