Being a developer I've never thought of color palette as playing such a vital role in online user experience. But this excerpt from Paypal helped me in rationalizing why I see the use of so many common colors on the web.

Color choice basics to consider when planning the palette of your website.
• Use colors from nature. Using forest greens, sea blues, and the reds and yellows of flowers and fall leaves is much more alluring to viewers than harsh colors not seen in the natural world (such as shocking pink, electric blue, and neon green).
• Color preference differs from men to women. Men prefer blue and orange while women are drawn to reds and yellows. Women are also capable of perceiving a much broader spectrum of colors and sense subtle shade variations much more easily.
• Viewer age plays a role in color choice. Young adults and teens are often excited by bright, vibrant colors and contrast. Older people tend to be more interested in websites that have more conservative color schemes such as blues, grays, and browns, without glaring contrasts that can tire the eye.
• Culture and nationality affect response to colors. In the United States and most European cultures, the color white symbolizes purity and marriage. However, in Japan and China, white is a color of bad luck and red is the traditional color of new brides. If your website is aimed at attracting international customers, make sure to research the cultural color beliefs of your target audience.
• The universal safe color. Blue seems to be a universally safe choice among different cultures, ages, and genders, evoking feelings of peace and calm for most everyone.

Choose background and text colors for good contrast and readability. The best choice for readability is a white background with black text. Other good choices include gray or black backgrounds with a well contrasting text color. Avoid harsh contrasts such as red and green or blue and yellow, which have been shown to cause eye fatigue.