Here's an example half-sentence from Collins
...wallpaper with stripes of dingy yellow.
Here's an example sentence from Cambridge
Her hair was a dingy brown colour.
Why do they use an article in the second instance but not in the former?
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Sign up to join this communityThis question is about how to follow the rules of countability with colour words.
The word "colour" here is a countable noun, so we have to use "a".
With actual colour words like "yellow", "red", etc., things are a bit different. They can be treated as countable or uncountable, with slightly different meanings.
If you treat a colour as uncountable, as in your first example, it refers to the range of colours included in the word as a mass noun, rather than one specific tone.
Colour words can also be used in a countable way. This refers to one specific tone of the colour, not the entire range:
...wallpaper with stripes of a dingy yellow
So in this example, the stripes are of a specific variety of yellow, which happens to be dingy.