The area around the scrotum is slightly swollen and reddish after surgery - swollen because there is an incision in the area, and red to purple due to local haemorrhage (bruising) into the surrounding area.
In most cases haematomas at surgery sites occur due to excess movement post-surgery, which is what your vet told you. However, they can be triggered soon after surgery, if the dog is running around his kennel, for example, or gets too excited or is lifted up on the way out the door! They usually happen slowly and would not be noticed for the first day or so.
Initially haematomas aren't opened and drained. There is a risk that the blood vessel has not sealed off, and making a drainage hole would cause more bleeding. Small ones will also resorb by themselves (large ones too - I've seen a 20L haematoma resolve to a 5L haematoma in a week).
As lankeela says, the swelling and redness will disappear with time. If the swelling gets worse, or your dog gets sick/dull/not quite right go back to your vet.