heat - Why are some of the biggest stars known blue?

My question refers to an overview of the biggest stars we know: http://farm5.static.flickr.com/4138/4820647230_faba1c9f3b_o.jpg

Why are some of those blue?

Answer

dmckee's right: the picture you link to shows stars, not planets.

The color of a star is almost entirely determined by its temperature. The light coming from a star is, to a good approximation, blackbody radiation (except for absorption lines in its spectrum, which are very important tools for learning about the star but have little effect on its color). The spectrum of a blackbody depends on its temperature, in such a way that it shifts from longer to shorter wavelengths as the temperature increases. So hot stars look blue, and cool stars look red. Even cooler objects, such as you, don't glow significantly at all in the visual part of the spectrum but do in the infrared.

At the moment, we know little or nothing about the colors of planets other than those in our solar system. Extrasolar planets are detected indirectly, via their effect on the star they orbit. They are not yet seen directly themselves.