You shouldn't assume that just because decidedly imperfect humans are incapable of truly observing the pinkness of IPU that it is not, in fact, pink. Just because we cannot make a judgement regarding the pinkness of IPU does not mean that such a judgement is impossible. We can, however, surmise that IPU is in fact pink, as it has not pointedly protested that conclusion.
We may, however, be mistaken on the nature of IPU's 'I'. It may, in fact, be both intangible and invisible, making all attempts to identify IPU as such by physical examination impossible. Despite the fact that the IPU then would be both invisible and intangible, and that it has never interacted with the world in living memory, it nonetheless must exist. The IPU, by definition, is both the most invisible and pinkest of all unicorns (note the capital letters!). If it can exist in our understanding, it must then exist in reality, as things which really exist are greater than those which conceptually exist, and as the most invisible and pinkest of all unicorns, IPU's existence in reality is necessary, as it would otherwise not be the most invisible or pinkest of unicorns.
Re: IR scope: The term 'pink' specifically refers to a hue of red, part of the visible light spectrum. IR radiation, by definition, is that just longer in wavelength than 'red' light, but shorter than microwave radiation. Therefore the IPU is in fact pink (a shade of red), and would be observed as such were it not invisible.