The chromatic scale is unlike any other bass scale. It consists of all 12 notes of the octave, played in order. You're unlikely to use a chromatic scale in any songs, but playing a chromatic scale is a great way to get familiar with the notes on the bass and get to know the fretboard.
Unlike other scales, there really is no root in a chromatic scale. Since every note is part of it, you can start playing anywhere. Even so, you will still hear people name a note as the root, for example "E chromatic scale." This simply means that you begin and end with that note, even though it holds no special role in the scale.
On the bass, there are several ways you can play a chromatic scale. Let's look at each one.