Stamping multiple times builds depth. It appears that more than one ink is used but this is not so. By stamping off and stamping over builds layers of light and dark ink tones to complete the image.
Choose one ink colour. 'Stamp off ' onto scrap paper and then stamp again without re-inking. The result is lighter in the ink shade. So that is how the different shades are achieved. With dark ink shades you may even stamp off three times to achieve the desired lighter tones.
I found that it is best to work from dark to light in ink tones. It is much easier to line up when stamping. I also found that stamping the lest detailed stamp first, then to next detailed stamp and so on. Looking at the baby bear stamps opposite, stamp in sequence from top to bottom. Same with the bow.