I want to do a tattoo of the koi fish. Is there a color which represents a grandfather?

Well not really. And that depends on what style of tattoos you like. Traditional. Or old school. Anyway play it safe. If it's to represent YOUR grandfather. Take a box of crayons and dump them out. Which colors make you think of your grandpa?? Your tattoo should mean what you truly want it to mean. Take this to the heart. I'm covered.