Blue hair: Tsunami is blue in the series, and that includes the top of her head, hence the color! I also decided to put it into a ponytail because I felt like that would match the idea of her swimming often, even if she was a human.
Dark blue wetsuit: Same reason as the blue hair. I decided to make it specifically a dark blue because I looked up examples of wetsuits, and many of them were black or other dark colors, which gave me the impression that most wetsuits are darker colors.
Blue sides of the wetsuit: Many wetsuits have lighter sides, so that's why I chose this detail.
Very pale stripes: These are a nod to her SeaWing bioluminescent scales! I thought this would be a nice touch :)
Hair gradient: This wasn't originally planned, but I realized how close the hair color and wetsuit sides were, which caused me to add this gradient so that it wouldn't look like a very crooked stripe. It was also my first hue-shifting experience, because I knew I would have to shade it, which is very difficult when you're picking the hues by hand.

(Base by Joy Ang, coloring by APeregrineInFlight on Fandom)


(Art by Mike Holmes)

The above pictures were found on the Wings of Fire Wiki page for Tsunami.

Wetsuit references:

(Image source: Amazon - yes, it's a kids wetsuit, it still helped me get a basic idea of what a wetsuit should look like)


(Image source: Rip Curl)

For these images, I just googled "wetsuit" and went to images.