Either color can be an indicator. In a non-modified state, the coloration pigments (anthocyanins) are not overtly visible and have (typically) a clear to yellowish color. I.e. you would not know they are there.
The first process in achieving a coloration that is visible is Glycosylation, in which sugars are added to the cell containing the non-colored anthocyanins. This converts the pigments to a "RED" color. Pigments that have only been glycosylated will remain red, but CAN have the sugars remediated and return to a non-colored state (if conditions are such that pigmentation is not needed).
The second process in coloration is Acetylation, in which the cell is further altered from visibly red pigments. This converts the pigments to a "Purple" color. Any pigments that have been acetylated are permanently colored and cannot be "downgraded" back to red or the original clear /yellow. (unless those pigments are broken from their containing cell and exposed to air, which would revert them back to red).
With MJ plants, these colorations can show on leafs, petioles and main stems; with petiole coloration being most prevalent. It does require energy for a plant to produce colored pigments and as plants don't waste energy; there has to be a return value for the plant to do this. As such, there are MANY cases where a plant may adapt to this coloration (so, not only with senescensing leaves). For instance, quite commonly (indoors) healthy leaves develop purple petioles to support both the movement in AND out of the leaf tissue; resulting in less loss to oxidation during trasnport (oxidation from light). Here it is of value for the plant to protect the nutrient movement into or out of a robust leaf for nutrient use efficiency.
Specifically with senescence of the leaves, typically a plant will have already fully acetylated the petiole before remediation of any mobile elements out of a dying leaf. Some strains may even spend energy on coloration into the leaf veins or entire leaf as well. If a leaf is fully purple then, excluding necrosis, there is no visible indicators for things like chlorophyll content dropping as a result of remobilization of nutrients in that leaf. In this case, a purple leaf may go (visibly) straight from purple to dead, lol.
