Focusing on the negative keeps us from being grateful about the positive.
After everyone in the room has had a sufficient time period to look for everything in the room that is red, ask them to close their eyes. Then ask them to keep their eyes closed and tell them they can raise their hand if they can name the yellow items in the room.
Most will not be able to name anything unless they are wearing the color themselves. Of course any color can be used.
Does that mean that the color did not exist when they were looking around? Of course not.
Explain that our mind when focused on something particular has a hard time concentrating on anything else. If we are always looking at all they negative things in our lives and all of our problems (red) we will probably miss all the things to be grateful for.(yellow)