Researchers can be incredibly dull sometimes.  If you were eating pringles and came across one that was colored would you eat it?  Would it mater if it was red, blue, black or orange?  No.  People don't eat things that don't look like they are supposed to.  So if you have a contol group with no colored chips and another group with some colored chips mixed in, well which group do you think is going to eat more?  Clearly the group without tainted chips will eat more.