Step-by-step explanation
There are 6 ways to choose the collar of the first cat.
There are 5 ways to choose the collar of the second cat, that is, 6 choices minus the color for the first cat.
There are 4 ways to choose the collar of the third cat, that is, 6 choices minus the colours for the first two cats.
Then, there are 120 possible selections if color repetitions are not allowed.
Answer
There are 120 possible selections if color repetitions are not allowed.