Ideally, the Denver Broncos will decide that they can't count on Drew Lock to develop into a Franchise QB and hence be willing to give the Lions pick # 9 for Matthew Stafford. That potentially could drive the price up for Stafford.
The Colts have the salary cap space to be able to afford making the Lions an offer for draft picks that would land them Stafford. Hence, the Colts can use free agency to help fill some needs more easily than most teams.
The Colts, which likely have serious interest in trading for Matthew Stafford, are almost certainly not in the mix for Deshaun Watson, because they are AFC South opponents of the Texans.