Ultraviolet range of the spectrum is typically invisible (for humans) but when viewed under a UV light anything that contains phosphors will glow. This is because phosphor absorbs the UV light energy and then re-emits it as visible light which we can see. Anything that appears black under a UV light has no phosphor because all the light wave lengths are being absorbed (nothing detectable is reflected back to our eyes).
All the colors we see are NOT really a part of the stamp, they are light wave lengths being reflected off the stamps surface. This is why ambient light is by far the most critical part of any color discussion. If a stamp looks black to us it means all the light wave lengths are being absorbed by the stamp. If the stamp looks white to us then all the light wave lengths are being reflected off the stamp. If a stamp looks red to us then the stamp is absorbing all the light wave lengths colors except the red wave lengths. The ambient red wave lengths are what we see as red.
Edit: How important is ambient light in determining the colors we see? Every time you walk into a grocery store, ambient lighting is being used to influence your purchasing decisions. Grocery store have long understood how ambient lighting impacts sales so they manipulate the ambient lighting over things like produce and meats. By making the apples look redder and or the lemons look more yellow they increase the likelihood of shopper buying items. Here are the wave lengths they manipulate in the store to influence the colors of things in the display cases in their stores. (Note how they avoid anything close to natural daylight.)