If he's got colored or LED turn signals, it wouldn't be an issue. I've actually thought about doing this to mine for daytime visibility. When the sun's on the taillights the right way, you may not see it get brighter if it's amber, but you usually see it change color if it's white. I've owned a few cars that had white housings and yellow bulbs, and even following behind them would always see it change color so long as the bulb coloring had not faded. I can't say the same for cars with amber housings.