The Color BlueI am a fan of looking at various ways symbolism shows up in the anime I watch and games that I play. Aqua is heavily associated with the color blue, whether you consider her hair/eyes, her outfit or her Wayfinder color. First and foremost, blue is often a representation of water, which is no big surprise here considering Aqua's name. Aqua would have the color blue for water while Terra has orange for land and Ven has green for the sky (green always reminds me of wind). Aquamarine especially tends to represent water.
Going deeper than just simple connections, though, blue also symbolizes peace, tranquility, calmness, stability and confidence among other traits. All of these words remind me strongly of Aqua's personality. She is very calm and collected, prefers to avoid fighting or violence of any kind and is a reliable and stable person that has confidence in herself. Blue can also represent unity and loyalty which are another two important things for Aqua. She is the person that tries with all her might to keep her friends together. She is the one who makes the Wayfinders that the three of them rely on for reminders that they are always together. She represents the unity between them. Aqua is also extremely loyal because she will do anything for her friends, no matter what the cost. They always come first.
Blue can symbolize healing, acceptance, love and comfort. I think these traits speak to Aqua's role as a caretaker. She clearly looks after Ven with love and comfort. I believe Aqua also would share these same traits with anyone in need, though. She is very caring and willing to extend a helping hand whenever she can. Her personality and atmosphere seems to be healing in nature because she is so warm and helpful.
Blue can represent not taking change very well. Aqua certainly takes a long time to change her black and white view of the world and it is only after many times of being challenged by people and situations before she finally realizes there are many shades of gray.
I've read that dark blue represents strength and power whereas light blue can be something considered more feminine. Aqua has dark blue in her outfit, possibly representing her strength as a Keyblade Master and as a person while her lighter blue hair and eyes can represent her more gentle and feminine qualities.