No it's not true. Garlic does nothing for a fish's colour. If you want to improve the colour of your fish, feed them a varied diet and give them the correct water chemistry for their species. Have a dark substrate and a picture on the back of the tank to help improve their colour.
Foods that can improve yellow colour include paprika, apricots, pumpkin, carrots and orange squash.
Foods that can improve red colour include things like beetroot and red capsicum (peppers for the Americans).
Foods that improve blue colours include purple carrots, black plums, blueberries.
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Some people say garlic improves a fish's appetite. I have not seen any scientific evidence that it does. Some people also say garlic is useful for deworming fish, again I have never seen anything to support that.
What I have seen is fish dying from eating onions and onion relatives (spring onion, leeks and shallots). It's not common and more often seen when people give onion or onion juice to fish. Garlic is related to onion.
If you want to try garlic you can but I don't recommend it. If you really want to try, you would crush the garlic up and soak food in the juice for a few minutes before feeding to the fish. You could also blend up some garlic into a powder and mix it with fish food. But there is a chance you could harm the fish.