I thought this might be alternate eye color as I was sure Shimakaze did not have purple eyes. Upon further review however, it appears her eye color is fully inconsistent: her in-game poetrait has grey eyes, but in other official art she has distinctly brown eyes, and fanart regularly makes them blue or green.

Given most of the art is fan made it's not surprising that there could be color shifts like this in her eye color versus her canon brown or grey. Especially as the common trope of eye color with blondes would be blue or green. It's also not surprising some of the art matching the canon colors, particularly when they're drawn kind of murky, being interpreted as a different color, in this case grey to blue and brown to green. Then add in artists wanting to make the colors more vibrant, end up with vibrant green or blue eyes.