The scourge marks are not "lash" marks but rather the WOUNDS left by a very nasty weapon, the Roman Flagrum. Modern day readers of this event tend to think in terms of the movies and the stripes laid on a bare back by a whip, or a cat-o-nine-tails. These are nothing compared to the flagrum. The flagrum was a wooden handle with two or three leather thongs tied to it and either a bone, lead balls, or bronze dumbell shaped objects tied to the ends of the thongs. These marks left by a flagrum are DEEP.
For a series of lectures I once presented to my church, I constructed a flagrum. Mine had three fifteen inch thongs, each with two round half inch lead balls tied in the end of the thong.
To demonstrate the efficacy of this vicious weapon to my lecture audience, I used it with moderate force on a pine board. Each of the six balls was imbedded into the wood a quarter inch. If I used it with more force, it was difficult to remove the balls from the wood.
If this weapon goes this deep into a wooden board, how deep would it gouge into human flesh? The flagrum would break the skin, tear it, and sometimes gouge pieces out. Many prisoners did not survive scourging.
Studies of the scourge marks show that they are dark because although they are probably hours older than the crucifixion wounds, they are still bloodied, although not as freshly. These blood stains enhances their appearance on the Shroud.