An introductory phrase sets the stage for the main part of the sentence:
- After the meeting was over, he was exhausted.
- While she was getting ready for bed, she heard a knock at the door.
There are several types of introductory phrase. Sometimes you have to have a comma; sometimes you can choose whether to have a comma; and sometimes you should not have a comma.
When an introductory phrase is very short (less than four words), the comma is usually optional. But if the phrase is longer than four words, it is better to use a comma. Always use a comma if the sentence could be misinterpreted otherwise.
These items are available in black and white, red and yellow, and blue and green.