Why are animation movies or weather forecast broadcasts filmed with blue background ?

Chroma key is a technique for blending two images, in which a color (or a small color range) from one image is removed (or made transparent), revealing another image behind it. This technique is also referred to as color keying, colour-separation overlay, green-screen, and blue-screen. It is commonly used for weather forecast broadcasts, wherein the presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background.

Developed for film as a method of compositing, the blue-screen process quickly became the most popular technique for creating unusual backgrounds.

First, the actors are filmed against a blue screen. Then, with a little darkroom wizardry, the blue areas were replaced with a background screen, dropping the actors into any world the producers desired.

The same process soon applied to video and computers. With film the process is chemical, with video it is electronic and with computers it is algorithmic. Of the three realms, the algorithmic is the most flexible.

Why Blue? Can’t other colors be used?

Red, green and blue channels have all been used, but blue has been favored for several reasons. Blue is the complementary color to flesh tone–since the most common color in most scenes is flesh tone, the opposite color is the logical choice to avoid conflicts. Historically, cameras and film have been most sensitive to blue light, although this is less true today.
Green has it’s own advantages, beyond the obvious one of greater flexibility in matting with blue foreground objects. Green paint has greater reflectance than blue paint which can make matting easier. Also, video cameras are usually most sensitive in the green channel, and often have the best resolution and detail in that channel. A disadvantage is that green spill is almost always objectionable and obvious even in small amounts, wheras blue can sometimes slip by unnoticed. Sometimes (usually) the background color reflects onto the foreground talent creating a slight blue tinge around the edges. This is known as blue spill. It doesn’t look nearly as bad as green spill, which one would get from green.

There are some problems with blue-screen. If the actor wears something blue it will become transparent, poking a hole right through the actor. You need to be very careful with the wardrobe for a blue-screen shot. This caused some consternation on the set of the movie Superman, where the blue-suited hero had a tendency to disappear altogether. To fix this niggling detail, they made a special purple Superman suit. Before compositing the final image, the suit was color-corrected back to blue.