Which college for hypnotic invisibility?
 

As it says. By hypnotic invisibility, I mean the power of the Shadow “to cloud men’s minds so they cannot see him.” I’m wavering between Illusion and Mind Control

Re: Which college for hypnotic invisibility?
 

I would say it strongly leans towards mind control.

Re: Which college for hypnotic invisibility?
 

The standard GURPS magic system college is Illusion and Creation for a reason. Its illusions are real physical things, they're visible from infinitely far away, block light from behind them (and thus cast real shadows), visible to any light detector whether it has a mind or not, etc. They have nothing at all to do with men's minds. Some other magic systems are more ambiguous, but generally it is so much harder to adjudicate what a mental illusion can or cannot do than a mental one that most systems lean against it - for example if you are targeting a mind, why would there be any limits on how big the illusion could be or how far away it appears?

Re: Which college for hypnotic invisibility?
 

I agree this is Mind Control, as unlike an Illusion, it would not work against a target without a mind.

A larger target is harder to believe, as it should generally displace more stuff - a giant should leave giant footprints, block light and wind, shake the very ground with its steps, be more visible from anything reflective, and so forth. This requires the spell to have more "processing power" to trick the mind into believing it's real. This might mean hard limits on what a given spell can produce, requiring more skill to produce larger illusions, or simply having larger illusions be easier to resist, depending on spell/system. Distance limitations are harder to justify; you might be able to say that it's harder to render the image at proper proportions the further away it gets, as well as you needing to account for more stuff being in the way of it, but that's kinda stretching things (the reason it's harder for a video card to render things further away is because this means having to render more things, since at a minimum it has to render everything within the camera's sight).

Re: Which college for hypnotic invisibility?
 

Starting from first principles, I would argue that you could get a bonus for a distant illusion, since flaws are concealed by diminution.