These are larger than pustules and papules, i.e. more than five millimetres. They are usually painless when they are skin colored, but when the color changes to red or yellow, they become painful. They grow very quickly.
This is why you shouldn’t pop out a pimple:
Acne causes the skin to tighten around the opening of a pore, which traps the acne inside. The acne continues to grow and multiply but does not flow out towards the surface of the skin.
And when you squeeze a pimple, the wall of the pores gets injured.
The inflammation signals which are produced by the acne bacteria reach the bloodstream that creates more inflammation. And if you squeeze the pus and oil, the skin will start growing over the wound, which will trap the bacteria inside. And instead of a nasty pimple, you will soon have a nasty nodule or cyst on your skin. Therefore, it is not a good idea to pop your pimple.