The green light bulb in Lucy's lamp is meant to make her think she's turned green but she immediately notices that Fred and Ricky look green too. Ricky then makes sure Fred plays along and say they don't look green to each other when they clearly do. In the next moment Ethel comes in and sees that Lucy appears green, but doesn't say anything about the others looking green. Odd considering Ethel isn't in on the boys' prank and she should have just as easily noticed that everyone in the room is green just like Lucy did.