How can old blood be red?

First, it’s generally accepted it is real blood on the shroud.

From a forensic point of view, the blood marks are
macroscopically consistent with blood, except for the
color which is too red for centuries-old blood.The “blood” is real blood: it does contain all the
molecular components of blood (iron, amount of iron
consistent with real blood, iron specifically attached to
heme, porphyrins, human serum including proteins, albu-
min, and finally high levels of bilirubin) as demonstrated
by microscopy, microspectroscopy, microchemistry and
immunology. … Shroud_Man

The redness of the blood is theorized to be caused by high levels of bilirubin.

The blood also contains very unusual high amounts of
bilirubin. This fact was not expected before the
experiments but can be understood if a quick and large
hemolysis occurred as should be the case for Roman
scourging.To explain that, Adler wrote that
the blood on the Shroud is not whole blood but exudates
(Adler, 1986) that left an imprint during the clotting
process on the corpse. In blood, almost all the potassium
is inside the red cells which remained on the body during
the clotting process. Adler (1986) wrote: “But a torture,
scourging and crucifixion leading to shock – that would
produce a tremendous hemolysis. In less than 30 s, the
hemolyzed hemoglobin will run through the liver, building
up very high bilirubin content in the blood. If that blood
then clots, the exudates forms, and all the intact cells with
hemoglobin stay behind, only the hemolyzed hemoglobin
goes out along with the serum albumin which binds the
bilirubin…. The blood has no cells, is very low in
potassium and has the right color and composition for the
blood of a man who was severely flogged and crucified”. … Shroud_Man