Sailormercury::Colors

Senshi Color//Blue:
Blue is a primary color. Its complimentary color (opposite color) is orange.

Mercury's eyes and hair are blue. The majority of her senshi outfit, such as the skirt, bows, and collar, is blue. As Ami, she often wears blue clothing.

Symbology:
Blue symbolizes calmness. During battles, Mercury is able to stay calm and collected and try to analyze the problem.

Blue is often associated with sadness or lonliness. Ami had difficulty making friends before meeting Usagi. She felt lonely because her dedication to school often isolated her.

In Asia, blue is often associated with self-cultivation and wealth. Ami works hard to improve herself, particularly in academics. She is a dedicated student and excells in whatever she puts her mind to.

Water and ice are often pictured as blue. Mercury's main attacks generally involve some form of water (water, ice, snow, bubbles, etc). Also, some of Mercury's attacks mention "aqua" which is another shade of blue and also a word for water.
Ami is a Virgo and this astrological sign is often associated with a pale blue color.

Some of the color information came from: Princeton Online