The Unicorn has green eyes and main camera. Banshee's are red.
I think you're probably referring to the Final Battle Banshee where everything turns "green", where in the novel the "green" psychoframe is actually described as rainbow coloured. Remember that yellowish-green aurora that pulled Axis away from Earth in CCA? That should be it, although it is visually depicted as bluish-green in the novel art and the MG kits follow this quite accurately.
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From what we've seen so far, the different variations and labelling of UC kits are:
Unicorn ver Ka - novel version, no vulcans, crap knee joints, pink beam sabres
Unicorn ver Ka Titanium finish - different paint job
Unicorn ver OVA - head vulcans + blue beam sabres, slightly better knees
FA Unicorn ver Ka - misleading nomenclature as it's actually a ver OVA Unicorn with the additional arsenal and green psychoframe
Banshee ver OVA - OVA version Banshee with fin funnel beam launcher + claw
Banshee Final Battle - Green psychoframe Banshee without OVA weapons
Out of all these variations, I think the Banshee OVA would be the best pick since 1) Banshee looks much more badass 2)
You'll probably get the usual RX-0 weapons in additional to the OVA ones, more goodies = more funthe bolded part is probably the reason why Banshee OVA is more expensive than Unicorn & Banshee Ver. Ka