The most common and most likely answer to this crossword clue is the 9 letter word NEGATIVES
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The straight part
We know that "Reverse-colour pictures" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reverse-colour pictures = negatives.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NEGATIVES mean?
NEGATIVES noun- Refusal or withholding of assents; prohibition, veto
- A right of veto.
- An image in which dark areas represent light ones, and the converse.
- (grammar) A word that indicates negation.
- A negative quantity.
- : A rep performed with weight in which the muscle begins at maximum contraction and is slowly extended; a movement performed using only the eccentric phase of muscle movement.
- The negative plate of a voltaic or electrolytic cell.
- To refuse; to veto.
- To contradict.
- To disprove.
- To make ineffective; to neutralize.