There is no official or traditional color for someone's 75th birthday. Recommended colors are those that the birthday honoree favors or shades that go well with the party theme. Some people believe that platinum is the appropriate color for a 75th birthday, but platinum is actually the traditional shade for a 70th anniversary.
Since the 75th birthday is a momentous one, bright, fun colors are appropriate. Once a color scheme is chosen, one should select the dishes, party favors, and decorations in the chosen colors. One fun idea is to have the birthday man or woman dress in one color and have the party goers dress in other colors so that the honoree stands out.