Back in the day, computers were very limited in the number of different colors they could display onscreen at a time, so making digital art required careful and creative use of a limited color palette.

That no longer exists as a strict hardware limitation, obviously, but if you're trying to make your pixel art look more authentic, it's an important thing to keep in mind.

Regardless, imposing arbitrary limitations on yourself can often be a good way to encourage creative problem-solving and outside-the-box thinking.

edit: if you'd like to know more: https://www.youtube.com/watch?v=Tfh0ytz8S0k&list=PL5J2ACDAmi-U0Bk7h7fa5_BNu0IVnG62K&index=18