Search for colours in the rainbow and you get many answers like this one:
https://answers.yahoo.com/question/index?qid=20080603071125AAMEsSq
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back when (whatever his name was) discovered the 'rainbow' he was living in a very religious society and so instead of concluding he found 6 colours in the rainbow (6 being the devils number), he said he found seven (a good religious number) 3. try looking at rainbow and finding the blue, violet AND indigo colours!!