[Idle musing]Being fair, though, I think the rational actor (or at least rational observer) would necessarily find that the pinkness (or lack thereof) of the Invisible Unicorn is actually irrelevant. If you don't have a measure of pinkness (i.e. the object is invisible and its color cannot be observed by your methods of observation), then the pinkness, insofar as the burden of consideration re: the actor is concerned, functionally does not exist. You can't say or see anything about it, it has no way of impacting your decision making, etc., so forth, so on.
The appropriate thing to do is disregard the light wavelength and either start molesting the unicorn to figure out what shape it is or pull out an infra-red scope or something. Appeals to the pinkness are irrelevant until you have a means to identify the pinkness. That doesn't necessarily mean you don't have other (perfectly acceptable means of justification) ways of identifying the unicorn.[/idle musing]
Point being that appeals to justification beyond light wavelength as to the nature of the unicorn is preferable to only justification by nature of light wavelength, and if the rational actor lacks the latter then they must turn to the former and, should there be no former, note they have no justification and find something more important than unicorn ogling to do with their time.