[[1]] is the colour of a universal indicator when it goes in a liquid with a pH of 9.

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Detailed Solution

Concept: The universal indicator will become blue when placed in a liquid with a pH of 9, indicating that it is a weak base.
The term "universal indicator" simply refers to a mixture of various dye colours that may be used to determine the pH of any substance by observing how the colour of the mixture changes based on the substance's pH and whether it is acidic, basic, or neutral. It is also referred to as the pH indicator as a result.
Since a base with a pH value of 9 belongs into the category of bases with pH values more than 7, the colour shown by the universal indicator when dipped in the liquid will be near to purple and will give a blue colour since it is a weak base rather than a strong base.
Hence, the answer is blue.

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