A pink contract is an agreement between an email spammer and the spammer's Internet service provider. The contract exempts the spammer from the provider's terms of service, which typically prohibit spamming. In return, the spammer pays far more money for his internet connection than non-spammers. AT&T got in trouble for such a contract.
The contract is called pink because that is the color of SPAM (the food), alluding to the fact that the contract enables spamming. 
"Any ISP in the United States can get in real trouble for signing pink contracts. Problem is that there's a whole big world wide web out there and the bulk of the pink contracts are signed with overseas ISPs where US Federal Authorities have absolutely no power."