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We describe colours when they add meaning to an image. The most common would include:
We only describe colour if when it adds meaning to the image. Remember, Alt-text replaces an image with text. It is a common mistake to presume people with print disabilities do not care about colour.
Most often, you will be describing the colour of the sky (pale blue, grey, dark black with white stars, etc.) or the colour of a person's clothes and hair (a blue polka dot t-shirt and blue jeans, or black curly hair, etc.).
For a list of colour related words please see the Alt-text Glossary.
There is no way to know exactly what a colour is when an image is in black and white. There is also no reason to have to describe the full black to grey to white scale across all the images.
When referring to colour in a black and white image stick to the following descriptors:
If you are describing something like the sky in a black and white image, avoid colour unless it is a night sky. Use descriptors like:
When in doubt, post the image and your question in the Alt-text Q&A!