Achromatic colours are not actually colourless. They're just colours without hue, or more simply, without colour. They're often used for backgrounds or textures to add interest without drawing attention away from the content.
The most common achromatic colours are:
Achromatic colours are not just limited to neutral colours. You can also use them as a background colour with a small gradient to give it a more interesting look.
For example:
Achromatic colours are also useful for creating a sense of balance and calmness in your design. They're often used in minimalist designs to create a sense of serenity.
But don't just take our word for it! Read more on monochromatic ideals or Neutral color theories.