Crossword Genius

Red, green, blue or black card? (7)

Ross

I believe the answer is:

oddball

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This clue could be a double definition.

'red green blue' is the first definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'black card?' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Maybe there's a link between them I don't understand?

'or' acts as a link.

Can you help me to learn more?

(Other definitions for oddball that I've seen before include "Eccentric 32" , "Crank" , "Weirdo" , "Eccentric character" , "Eccentric person like peculiar sphere" .)

I've seen this clue in The Independent and The Telegraph.
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