There is no specific use of "a' with adjectives of colour.
"a" cannot be used with an adjective alone.
You say: it is a good car.
You say: it is good.
You cannot say: it is a good.
In your example sentences, 3 is correct, 2 is incorrect, as per above.
Sentence 1 is a specific use.
In 1, you would not use the article to say "the wild cat is grey".
In 1, "a" reports directly to the adjective "brindled", to indicate possible varations (brindle is a "brownish" colour that is not as clearly defined as "red" or "green").
This is not limited to colours:
- "the shape of this cloth is a rough square" (slightly undefined shape) but "the shape of this cloth is square" (clearly defined).
- "the film runs for an approximate two hours" (slightly undefined time) but "the film runs for two hours" (clearly defined).