Pink Pearl

Color

Lucia's theme color is, like most magical girl heroines, pink. At one point in history, pink was acknowledged as a masculine color, as pink was associated with blood (which is linked with strength, obviously). However, the situation changed around the time of WWII, and pink is now considered a feminine color in many cultures. Japan does not seem like an exception, as very girly shows like Pichi Pichi Pitch usually have a character, usually female, associated with pink and/or features a lot of pink. Along with femininity, pink can denote calmness and tenderness. Also, this might be slightly trivial, but the pink rose is a symbol of friendship.

For the sake of discussion, here are a few descriptions of the color pink:

Pink is a combination of red and white. The quality of energy in pink is determined by how much red is present. White is the potential for fullness, while red helps you to achieve that potential. Pink combines these energies. Shades of deep pink, such as magenta, are effective in neutralizing disorder and violence. Some prisons use limited deep pink tones to diffuse aggressive behaviour. - Emily Gems

Light red represents joy, sexuality, passion, sensitivity, and love.
Pink signifies romance, love, and friendship. It denotes feminine qualities and passiveness.
- Color Wheel Pro

Pink is a very suitable color for Lucia. Lucia embodies the many traits emulated by the color, such as kindness and peace. She is not the type of character to hold grudges against people and, in turn, can even befriend the bitterest of enemies. Lucia also gives great importance to her loved ones, to the point she could even sacrifice her own life for them.

The only quality I disagree with in the descriptions is passiveness. Time and time has shown that Lucia is anything but passive. Lucia is a strong-willed and determined young lady. If she wants to or needs to get something done, she will get it done. Had she been passive, this show would not even exist, as Lucia would not have pursued Kaito, nor would she have defeated the main villains in the series.