The first thing most people notice about Hotaru is that she wears a lot of black. Most people view black as the color of depression and sadness. Hotaru is a very troubled individual so she might wear black as a way to express her pain. Her lily-white skin is contrasted against her dark hair and the dark clothing she wears, which gives her a slightly 'morbid' appearance.
But black is also the color of elegance. Hotaru comes from a fairly rich and well-off family, and her choice of clothing often reflects this. In the anime series, she often wears plain long-sleeved short black dresses; except when she goes to Mugen Gakuen High School, then she put her school uniform. After Hotaru is reborn, she is shown as a younger child, and her fashion has changed. In the later seasons of the Sailormoon anime, Hotaru can be seen wearing bright colors like pale blue and pink. Perhaps this is a symbol of her transition and rebirth; she is no longer a miserable individual haunted by the presence of Mistress Nine, so she no longer feels the need to dress in sad colors.