It looks like the Colts figured out who they are adding to the squad to fill the two open roster spots. Daniel "Boom" Herron will be joining the Colts as an addition from the Cincinnati Bengals practice squad at running back. And free agent Andy Studebaker who was released by the Kansas City Chiefs will be joining the Colts as an addition at linebacker.
I'd heard news that the Colts had been looking at an RB from the Detroit Lions. But I think Herron is likely a much better addition. The Bengals usually have a lot of quality depth at running back. Even the RB's on their practice squad are usually quite good. Herron also played for Ohio State and Ohio State normally has some pretty descent running backs.
The reason the Colts likely brought in a linebacker like Studebaker is to fill in for Bjoern Werner who's out injured. For more info on the two players there's an Indianapolis Star article on the players. Here's also an NFL Draft Report on Studebaker.