Chromatography is a process whereby different fractions of a liquid spread out by different amounts on a substrate. Children habitually place M&Ms on a piece of absorbent paper, and by dripping water on them, the constituent colours spread out, giving, say, rings of blue and yellow when the original M&M was green. Many liquids behave in a similar way, without needing extra solute to separate them, as they are runny anyway, although they dry before distinct rings are formed. In this case they tend to stay roughly the same colour, with a distinct 'halo' of one of their constituents around them. I'm speculating that a drop of blood on a piece of cloth, be it dripped onto cloth directly from a wound, absorbed into a covering of cloth over a wound, or dripped by a forger from a pipette, might have been assumed by Barbet to have demonstrated a similar process. However, he did not actually experiment to find out if this assumption was true.
When one applies a dressing to an excoriation, which appears to have happened here
[Caution, not for the squeamish] (
http://homelineimpact.com/wp-content/uploads/2013/02/Homeline-Enluxtra-use-6.jpg) a large amount of yellowish serum has been absorbed by the cloth, and associated with it a reddish liquid which may consist of haemolytic products, and in the middle and indistinct mass of old blood cells and clotting material. Although the scourge wounds are considerably smaller, this is the sort of stain they ought to have produced. There is no evidence for that sort of stain on the shroud. In particular, I think that if the dressing was observed under UV light, it would glow very brightly, with minor extinction towards the middle. It would not show a distinct dark image of the wound with an almost indistinguishable greeny-blue glow round the edges.