Origin of the Title "Huang Di"
"Huang Di", a name addresses by people in ancient times, was originated from "San Huang and Wu Di", means three emperors and five monarchs. The three emperors refer to the Emperor of Heaven, Emperor of Earth, and Emperor of Human Being. They are three leading figures in ancient legends. "Di" in "Five Di" refers to the supreme dominator of the universe known as Tian Di, having supreme power, is one of the five monarchs. The rest of the four honored themselves as Xi Di (Monarch in the West), Dong Di (Monarch in the East), Zhong Di (Monarch in the Middle), and Bei Di (Monarch in the North) respectively, when chaos happened due to war among different countries. In this way, the term "Di" as a legend appeared to the world and became a divine name of king. Some people say that "Di" refers to Huang Di, Yan Di and Chi Yao.
When Ying Zheng, the founder of Qin Dynasty, unified the whole country, he considered himself to be the one who made larger contribution as compare to "San Huang and Wu Di", the three emperors and five monarchs. And thus, he privileged himself with a combined title of "Hung and Di", i.e., Huang Di. After that, the emperor of each dynasty is named Hung Di.
Yellow Color and Emperor
Yellow color was considered as a symbol of imperial power by the people in ancient times. In fact, this idea was originated from the philosophy of worshipping of earth by the people, living on agriculture. According to the traditional school of Yin and Yang, the yellow color stands for earth, one of the five elements of philosophy. As this earth is at the center of the universe, so, it receieved more respect and honor.
Huang Di and the Number "nine"
In ancient China, the number "nine" was considered as the apex of Yang numbers, in other words, "nine" is the biggest number among singulars. Therefore, "nine" is often associated with emperors and the things related to them, for instance, the title or position of empror was Jiu Wu "九五", Jiu means nine in Chinese.
The other examples relating to number "nine", associated with imperial power, are: Bronze ware known as Jiu Ding; there were nine ministers around the emperor; there were Nine Grade Official Ranks; the Forbidden City has 9999 and a half rooms; Tiananmen Square has nine-rooms; the doornails on the door of Forbidden City, Imperial Garden, and Xanadu palace are all 81 in number, the product of nine multiplying by nine.
Whenever, there were celebrations of Emperor's birthday at large scale, there were 99 kinds of performances, such as acrobatics and lantern show, etc. and was called as 'Nine-nine Grand Celebration'. The number of tributes to imperial officials on their birthday parties or during Spring Festival was also counted by nine. But for the average person, including nobles or landlords, their daily necessities were, of course, never counted by number "nine".