Is it possible to create an illusion which can only be viewed by the caster? The idea is to tag targets with a variety of classifications, be able to see them while others cannot.
Used might be remembering names, targeting only your own grogs, marking somebody as important or dangerous in a crowd so they are easy to follow.
I'm thinking of a special CrIm effect to tag/change the target's image, and then a version of Discern Own Illusions to allow you to see the tags.
I've thought about changing the hue of a target's clothes slightly so they can't really tell but the second spell above would highlight it plainly, and then started to ponder the more advanced version which is almost like a HUD for magi.
My initial thought was "No. Answer done."
But then I realised there might be ways. The answer would strictly speaking still be no, because of the word 'viewed', but I suspect you could tag someone with a type of species that humans (read: probably anyone without Second Sight) could not sense, and then create a spell, giving you the ability to sense species of that nature, T: Vision.
You might want to limit the use thereof though, to limit Warping.
I was also thinking of the way species is perceived can be changed, e.g. so heat can be viewed as an image.
Perhaps a normally not perceivable noise either too high or too low in pitch (Base CrIm 1, plus 1 mag for being odd, 1 for moving, 1 for high detail, 2 sun) might be still viewable using a second spell affecting the caster's vision.
Second sight might detect the odd sound but not know how to reinterpret it as a signal of a significant property.
I also thought that some glasses or helm could be enchanted to allow the caster to see the species visually.
The second spell detects the sound species as odd sound and re-renders it as visual species created just in front of the wearer's eyes. It makes a more complex solution but also allows it to remain hard for others to comprehend.
Creating the species in front of the user's eyes avoids warping.
MoH's Persephone (p.87ff) has invented The Factor's Roll Call (p.92) for that purpose. It is InMe 20 R: Eye, T: Ind, and the way of setting the tag is indirect, clever, and simple, though not fool-proof. A maga could make that spell into a R: Self T: Vision InMe 35 spell, thus making the tag visible.
A maga could also enspell people by a CrIm spell with tags of arbitrary content, that only affect - say - the taste of their earlobes. And then cast an InIm R: Self T: Vision Synaesthesia spell (HoH:S p.64f), to herself see that taste and hence the tag's content. The purpose of this considerable use of magic is her's to determine.
Alternatively, a simple Creo Vim (base 1), Ind, Sun (+2), Sight (+3), 10, let's call it "Touch of Hermes". You simply put a magical effect on the target, with no other purpose but to taint it with a little bit of magic, yet weaker than the Phantom Gift so there is no side effect. Then an appropriate InVi will allow to detect it.
There is no guidelines for detecting personal spell similar to the one for your own illusion, but I believe it is reasonable to design such a spell, which would be of lower magnitude than a generic magic detection spells.
So I would go with "Discern Own Magics", InVi 15 (a copy-paste of Discern Own Illusion, with a Base 1 effect: detect his own magic).
Otherwise, it would be a Base 5 Intellego Vim spell, +4 for Vision, +2 Sun, so a level 35 (I removed the +2 for Techniques & Form, and bump the duration to Sun).
Second option, as you proposed:
You cast a minor illusion spell on the target (or on his clothes), and you redesign Discern Own Illusion, but instead of making your own illusions more transparent, it amplifies it - it is a cosmetic change since it does not change fundamentaly the functionality of the spell.
"The Faint Glow", CrIm base 1, duration Sun (+2), Sight (+3), for a final CrIm 10 (add +1, if you want to be able to target a Part - like a hat on a person), allows you to give a tiny glow or shade to the target. It is only perceivable in darkness or in strong lighting, depending on the selected effect.
Or cover them in an illusion which is an exact likeness of themselves (or something smaller, like an article of clothing), then use a spell like InVi to keep track of that illusion.
Or make a sign, make it invisible, and stick it to their back...
Creating a spell that creates poisons that only affect one person by touch or AC should be harder than designing a spell which only creates poisons which affect Bob. Both seem viable to me. The second is a tailored spell, the first needs to pay for a bit of complexity but seems ok.
(These would be Cr(Mu) spells, of course. In the case of the wasps, I'm probably better off with Cr(Re) so that the wasps do what I want. The Cr(Mu) version is at least one magnitude more difficult than the ordinary Cr spell, possibly quite a bit more than one.)
With Rego it's easy: It's a normal to use Re to command animals.
But I'm not asking about that: In the context of this thread, if I can create species that are only visible to one person using Mu, why not use the same technique to create wasps that are intangible to only one person, or only tangible to one person?
Why? If I can create real species that are only visible to one person, what stops me from creating real fire that only burns one person, using Muto, as with species?
(I could use Re to control where the species go, what the wasps attack, where the fire goes, and what they avoid. But if I use Muto, I get to avoid the inevitable Finesse rolls to maneuver all this stuff. And I might be deficient in Rego. And Cr(Mu)Im has been suggested, so I'm exploring the implications.)