If someone tells you to hold your tongue it means they want you to stop talking, because they don't like what you are saying.
I'm going to have to hold my tongue because I keep saying what I think and people get upset!
I had to tell Jane to hold her tongue; what she was saying didn't help us move forward in our planning meeting.
If you make a slip of the tongue, you pronounce a word incorrectly or you say something you didn't mean to say.
I meant to tell her she was fun not fat; it was just a slip of the tongue!
Chameleons, like the one in this photo, are able to change colour, though not all species can. In a few types, the colour change is used as camouflage, but in most it is a form of communication to attract potential mates or warn off rivals. They will revert to a default colour that merges with their habitat - green for a jungle species, beige for a desert one.