Let's say it's a hot summer day and you'd like to find a water bottle that keeps the water cooler. There is a lot of light around us which may warm the water up. Light is a type of wave. Just like waves on water, different colors of light have different wavelengths. For example, red light has longer wave length than blue light does. Some light has even longer wave length than red light which we cannot see, such as infrared or microwave.
Different things absorb different light. For example, a blue bottle reflects blue light and absorb any other colors of light. The light absorbed warms the bottle up. Water is transparent, meaning that it does not absorb any color of light. Nevertheless, it absorbs invisible light, such as microwaves significantly (this is how a microwave warms up the food!).
Now back to your question. Under the sun, a darker bottle absorbs more visible light, so it warms up the water faster. The color probably does not matter too much in the shade though. Because there is a lot of infrared around, it warms up the water regardless of the color of the bottle. An ideal bottle would reflect all the incoming light (including infrared), such as the thermos with shiny surface.
(published on 03/14/12)