為什麼表情圖案大部分都是黃顏色的？

比如QQ表情，微博表情，人人網的表情都是黃色為主。

The smiley was first introduced to popular culture as part of a promotion by New York radio station WMCA beginning in 1962. Listeners who answered their phone "WMCA Good Guys!" were rewarded with a "WMCA good guys" sweatshirt that incorporated a happy face into its design. Thousands of these sweatshirts were given away.[5][6][7] The WMCA smiley was yellow with black dots as eyes, but it had a slightly crooked smile instead of a full smile, and no creases in the mouth.[7]

In 1963, Harvey Ball, an American commercial artist, was employed by State Mutual Life Assurance Company ofWorcester, Massachusetts (now known asHanover Insurance) to create a happy face to raise the morale of the employees. Ball created the design in ten minutes and was paid $45 (equivalent to $330 USD in 2012 currency). His rendition, with bright yellow background, dark oval eyes, full smile and creases at the sides of the mouth,[7] was imprinted on more than fifty million buttons and was familiar around the world. The design is so simple that it is certain that similar versions were produced before 1963, including those cited above. However, Ball』s rendition, as described here, has become the most iconic version.[6][8]

http://en.m.wikipedia.org/wiki/Smiley

上圖。