What word is used to describe an absence of colour?

The most common word to describe an absence of color is achromatic.

Here are some other words that can be used, depending on the context:

* Colorless

* Monochrome

* Black and white

* Neutral

* Grey

* Pale

* Desaturated

* Tint (specifically for a very light shade)

The best choice will depend on the specific situation and what you want to emphasize.