Board Thread:Questions and Answers/@comment-137.148.39.151-20140116153728/@comment-4782885-20140117200545

It's said in Free! Guidebook interview: Hiroko Utsumi x Futoshi Nishiya about their swimsuit colors as well as some other interesting things about Free! characters.

-How did you decide the color for each character’s swimsuit? '' Utsumi: Each character’s swimsuit reflects their image color. However, Haruka’s eyes are blue, so we changed his to purple. In return, Rei obtained Haruka’s blue. Only those two’s colors changed.''

You can read all about it on these two links: Part 1 Part 2