mmm, gemini, where did you knew or find that answer? sounds true but this is what I got...

pencils painted yellow were sold as early as 1854, but yellow didn't become the predominant color for pencils until around 1890. it was at that time that the L&C Hardtmuuth Company of Austria introduced a pencil of such high quailty that it became the standard for others to follow. the pencil was named Koh-I-Noor after the famous Indian Diamond, and it was painted golden yellow, supposedly because, with it's black lead, that made it the color of the Austro-Hungarian flag. this pencil, which was advertised as "the original yellow pencil," was such a hug success that it established yellow as the symbol of quality in pencils. other pencils manufactuers soon began painting their pencils yellow, and today three out of four pencils sold are that color.

ever wonder why we knock on wood for good luck? good luck.