The stamp on the left paper appears to me to be toned, making it a bit more yellow. This 'background' hue can influence our eyes a bit when we try to compare the two stamps.
For example, the bottom image shown here is your right hand stamp color applied to your left hand paper.
Even though it is the same (right hand design layered onto the left hand toned paper) note how the blue now appears to be different color from the original right hand stamp. This is all due to the background color influencing our eyes.
To demonstrate how much the background color can influence our eyes, here is an image; the horizonal gray bar is exactly the same color across the entire face of it. Yet our eyes trick us into seeing it with a gradient, getting darker towards the right (it is not, it is the same color).
Gray bar shown here is exactly the same as the gray bar above.