Viridian Entente is not all green alliance. Few members are different color, so I doubt it would be absolute requirement.
HeliPilot should ask alliance leaders if he can change team color (I have already sent private mail for him about this).
If his alliance is that zealous about color, then I won't have any problem chancing my color.
(Err... Is there any sanctions of cancelling trade, changing team, and rearranging trade? Sounds too easy...)