Red, which is #FF0000 in hex code, can be shortened to #F00. That is, one digit for red, one digit for green, one digit for blue.

This reduces the total number of possible colors to around 4,000. But browsers will interpret #FF0000 and #F00 as exactly the same color.

<style>
  body {
    background-color: #F00;
  }
</style>