Does your project and API support or implements green addresses? I am looking for a green address implementation/API other than the
https://greenaddress.it one.
No, BlockTrail uses 2-of-3 multisig (unlike greenaddress which makes use of 2-of-2), so BlockTrail does not have the control over the coins to prevent its users from double spending (and thus does not provide "greenaddress functionality" between its users)
Generally I understand what multisig is for but could you explain in the case of BlockTrail service why need their signature for the transaction, what's the advantage of having a multisig transaction and BlockTrail's signature on the transaction?