User blog comment:Lia Schiffer/ANIME SPECIAL: 268: Hatred and Jealousy, Orihime's Dilemma/@comment-1248963-20100428075540

I know, it made sense when only the Espada had colored Cero, but lesser Arrancar having rainbow Cero... I love colors, but it makes Ulquiorra's eco-friendly (I read that somewhere and I laughed so hard) Cero less unique. I guess it was implemented since many Espada (Aaroniero, Szayel, Zommari, Luppi and Baraggan, nearly half of them) never showed their Cero. I would kill to see Szayel's cero, I bet it would've been pink.

On another perspective, even Grimmjow's Cero was colored red in the beginning, and until later was changed to blue, so maybe when the animators saw that Ulquiorra's green was more popular than the standard red they decided to give each Arrancar an individual color... who knows.