Board Thread:Questions and Answers/@comment-110.22.214.120-20161122091306/@comment-24947676-20161122111438

Miku's color is actually cyan.

Since Miku is cyan, the Kagamines are yellow and orange, and Luka is pink, this leaves open green, blue, red, and purple. (Meiko and Kaito aren't CVs)

I'd hesitate to use red or purple, since the CVs use pale colors, and using red or purple would be too similar to Luka. Which leaves green or blue. I'd hesitate to use blue due it being too similar to Miku.

So, if my guess is correct, I'd say green.