Our perception of colour is a function of how the pigments interact with the light that illuminates them. How an ink will appear under yellow incandescent light will be different from how it appears under daylight, which will differ still from how it appears under various forms of coloured light. However, usually a colour will appear more or less the way you would expect it to appear with the addition of the colour that is inherent in the light. Long wave ultraviolet light is of course, a purple light. So the appearance of most colours would appear darker and washed over with a purple undertone. I would refer to these inks as non-transformative, because the introduction of black light does not transform the colour from one to another, but merely modifies it by making it appear, darker, duller or brighter.
However, there are some inks used on the Centennial issue that are a completely different colour under the ultraviolet light from their colour in normal light. Quite often, that colour is black, but in many other instances, it is a different colour other than black. These inks are what I would call transformative, for this reason. So in studying the inks used for this issue, after you have considered the shade differences as they appear under normal lighting conditions, there is the question of whether of not the inks are transformative, and what colour they appear under the ultraviolet light. It is quite possible, and quite common, in fact, to find many instances in which two stamps that appear to be more or less identical under normal light, will appear to be different colours under ultra-violet light.
To illustrate what I am talking about, let us now take an example of three 1c stamps shown below:
Now, let's take a look at ink which looks slightly lighter, and/or brighter under the light. Take these four stamps:
Now let's take a look at them under UV:
This type of ink seems to be found on both perforations of the BABN booklet stamps, and many of the CBN sheet stamps, with the deeper, duller browns, both untagged and Winnipeg tagged.
Let's take a look at the following four stamps:
Let's take a look at how they appear under UV: