It's Time
It was pink.
Not only did his parents have this pink color on their threads but also his siblings. He concluded that the pink thread signified familial love after much consideration.
Other than the pink thread, they also had some blue mixed in with that pink, leading him to believe that it signified trust, loyalty or amiable relationships.
Because, he saw blue threads from the guards, Duke Evan Cauldon, Hendrickson, Randal and others. They displayed loyalty in their attitudes, making him believe the blue thread signified trust, faith, dependence, designating most amicable relationships.
He guessed that Drake would also have a blue thread connecting to him if he checked in the future... But that remained to be seen.
Then comes the black thread, which he could conclude that it basically meant many negative emotions showcasing hostility. He confirmed this point when he roamed in prison while interacting with the prisoners.