Your question prompted me to take another look at that painted cartouche, im not sure i can answer your specific question, but i have noticed something else that may or may not be of significance.
There seems to be creamy coloured marks overlaying the red ochre , and as ive mentioned before these stones look as if they had a plaster wash at some stage.
Hear is what Petrie says,
>Some of the lines in this chamber, drawn in red on the S. wall blocks of granite, are over some of the plastering, but under other parts of the plaster. These lines, therefore, were drawn during the building, and while the plaster was being laid on, and slopped like whitewash into the joints. The red lines are always ill–defined and broad, about ¼ to 1½ inch; but, to give better definition, finer black lines were often used, either over the red or alone,
Now ive taken a screen shot of that photo by Collett Dowel , and circled a specific area.
Now if you look very carefully you will notice some creamy whitish marks overlaying the red ochre marks.
These creamy whitish marks are of a different hue to the surrounding white plaster wash.
So if someone has altered the original cartouche and wanted to make it seem the cartouche predates the plaster wash , then by applying additional plaster over the altered red ochre would make it seem the cartouche is contemporary with the construction.
Take a long close look at the different hue to these small creamy white marks that overlay the red ochre.
But getting back to your question on the yellow stain, now there does seem to be more than one area that looks yellowish , one might conclude that the cartouche area has been repeatedly cleaned over time, by various interested parties, so maybe one yellow area is due to the plaster being worn away and is now showing the original limestone rock.