Simple
Fuku
Each of the senshi are represented, and are most recongnized for the color of their fukus. Sailor Mercurys fuku is different shades of blue, which is also based on her attacks, which are water related.
This is the original design for Sailor Mercury. It still has the blue color thing, and there are very few details that are different from the original one. But you can see that she has blue, transparent sleeves that go over her arms until the elbow gloves. The next is the little chain that hangs down from the brooch. In the anime, and the manga, that does not appear, and it stays as a normal, round brooch. That also brings up the point that the brooch is the same color as the bow, and it is very clear in the newer fuku that is a dark blue.