Mamizou also is a brunette and isn't quick to use force at all, though. There's also a few more brunettes like Kagerou, Hatate, Lyrica and Yatsuhashi, and I feel you won't be getting far grouping them.
A lot of these groupings seem incredibly farfetched to me, really. You say Ran doesn't seem too serious, but she's really one of the most serious characters in the series (so is Lunasa, another blonde). Parsee and Yuugi are pretty much polar opposites and... yeah, you see what I'm getting at; the blondes are really just too heterogenous to group them together distinctively.
Same for the others. You say Remi, Tenshi and Yuuka are nice and likable, but they really... aren't. At least, many other characters fit that bill much more, making them actually some of the least likely characters to have blue/green hair by your definition.
ZUN probably had his reasons for the hair colours of some characters (and I would imagine he just picked some of them at random), but it isn't something as simple as "servant = redheads".
Just for the hell of it, I have compiled a list of characters by hair colour:
Blonde: [Elis, Ellen, Kana, Chiyuri, Kurumi, Elly, Mugetsu, Gengetsu, Luize, Yuki, Yumeko,] Marisa, Rumia, Flandre, Alice, Lily White, Lunasa, Ran, Yukari, Medicine, Shizuha, Minoriko, Suwako, Yamame, Parsee, Yuugi, Shou, Byakuren (partly), Luna Child, Miko, Toyohime, Maribel
Orange/Ginger: [Orange?,] Suika, Sunny Milk, Kosuzu?
Red: [ShinGyoku Female, SoEW Marisa, Kotohime, Yumemi, Orange?,] Hong Meiling, Koakuma, Komachi, Orin, Sekibanki, Raiko?, Kosuzu?
Brunette: [Rika,] Reimu, Chen, Lyrica, Utsuho?, Hatate, Star Sapphire, Mamizou, Kagerou, Yatsuhashi, Raiko?, Renko
Black: [ShinGyoku Male, Konngara,] Tewi, Kaguya, Aya, Utsuho?, Murasa, Nue, Seija
Pink: [Sara,] Yuyuko, Mystia, Satori, Kokoro, Kasen
Purple/Lavender/etc.: [PC-98 Reimu, Meira, Rikako,] Patchouli, Remilia (fighters), Letty, Reisen, Iku, Byakuren (partly), Ichirin (UFO), Benben, Shinmyoumaru, Akyuu, Raysen, Yorihime
Blue: [Sariel, Mai,] Cirno, Remilia (main), Merlin, Keine (human), Nitori, Kanako, Tenshi, Kogasa, Ichirin (HM), Seiga, Wakasagihime
Teal/Turquoise: Wriggle, Eiki, Hina, Kyouko
Green: [Mima, Ruukoto,] Yuuka, Daiyousei, ex-Keine, Sanae, Kisume, Tojiko
White/Silver: [Shinki,] Sakuya, Youmu, Eirin, Mokou, Momiji, Futo, Rinnosuke, Tokiko
Grey: Nazrin, Yoshika