That does seem like the ideal end-state. She has no idea if it's possible, to feed your visual illusion as input something you can't distinguish visually, not sourced from any real source you're referencing but constituted backwards from what it'd look like through a prism. But it seems like the sort of thing that you might have to try for a long time before you could be sure it wasn't possible, and where you'd learn something interesting from trying.

Possible step one, overlaying two different light sources to make a new light that looks the same as some other light, but has a different underlying rainbow.

Mixing firelight and white-hot metal until they look the same color as sunlight, for example, adding more or less firelight to the mix, shifting the light's tone redder or bluer, until it looks the same as the sunlight-source.  Possibly, if that's a thing you can even do.  But trying to preserve the part where fire-light + metal-light should have a different underlying rainbow than sunlight.

Then, practice shifting back and forth between 'rainbow 1' and 'rainbow 2' while keeping the appearance of the light source constant.

She hasn't actually seen white-hot metal. She's seen a forge, but the metal was not heated to the point of glowing white. She'll see if she can get there with light sources she's actually encountered, though.

Somebody's probably got a spell for it?  It seems like the sort of thing that absurdly overspecialized-for-combat Golarion magic ought to do.

There's a spell that makes metal red-hot, which is suggestive but she doesn't actually know of a more powerful version that makes the metal hotter. Her understanding is that it is sometimes used in combat but was actually a failed attempt at a utility spell that could replace forging.

Actually, now that Keltham thinks about, he has a nearly white-hot metal sex toy.  Might not work because it's just an illusion itself, but then again, it might?  Or it might at least give her a light-source that could be combined with firelight to make something that looks sunny but has a different underlying spectrum.

That'd be great.

(Some people are staring. What. In alter-Cheliax that would be an extremely weird thing to overhear.)

Oh, stop it.  Do they have any idea how hard it is to do something that Carissa Sevar will find even slightly scary?

PL-timestamp:  Day 25 (21) / Evening

Would Carissa Sevar care to try to persuade him otherwise, before Keltham orders her off to his cuddleroom to cuddle?

No, she wouldn't.

Didn't think so.  Off with her, then!

Yep, definitely feeling closer to her now.

They just have to remember to work on common problems where Carissa can also be impressive, hopefully.

"Well, I'm not expecting to have a shortage of those.

Are you - okay? Happy? Satisfied with the path we're on? Stuff keeps happening and I just want to be sure that you feel like - like you're glad you're here, ideally, but if that's too high a bar, like you're growing closer to the true Keltham -"

"I am.  A little sad about the Korva thing, but - pretty happy about everything else.  There'll probably be additional hiccups and nothing will be as easy as it looks and everything will take longer than expected, because that's just always true, but - it feels to me like progress is being made, things are within reach, I'm successfully using magic to cheat, and I'll be able to make enormous quantities of acid."

She is possibly more reassured to hear that than makes strategic sense. "So much acid! I'm having a great time. But for me 'way better than anything else that was ever going to happen' is a much lower bar."

"Eh.  Not that high of a bar for me either, unless you count whatever dath ilan's Future would've had to offer me."

"...now I feel distressed at imagining the worlds where Keltham didn't die in a plane crash. I am ...pretty sure that is not a normal emotion and I assume I'm applying the theory of worlds wrong, since you haven't actually explained it except in bits. But what if those worlds exist and he's sad and lonely."

They definitely exist, yes.  Possibly most of the branching Kelthams identical to him as of the moment of getting on the airplane, are still alive in dath ilan.  People were saying at the end that they couldn't think of any better explanation for what took out the left wing besides a meteor strike, and there's no way Keltham-who-boarded-the-airplane could've been precisely state-correlated with a meteor strike that precisely pessimized.

But that is not what Carissa is asking about, not in the most important part.

"Nobody's hit him with an Owl's Wisdom yet.  He still has a dream, he's energized and pursuing it, that counts for a lot."

"When he gets older and realizes what's actually going on in his life, he might be less happy then."

"But - there's always the Future.  For everyone.  Except this version of Keltham, who is very rare, and even he - got you, and Yaisa, and the Project.  And someday, if Golarion can make its own version of Civilization, this place's own Future."

"I think we can do it. Maybe it'll be hard, sure, but - we've got forever, right, at some point we'll get around to it just because we were getting bored."

"It's not quite something I can promise you, Carissa.  If this place is wrong enough for me, in the end - if everything Golarion seems to be, in terms of how much it's suited for me, is misleading - then I might turn statue to wait on someone else making a Future where I can be happy, or touch the Starstone, or go to my god's afterlife, or, if all those things look not good enough for me, walk out of this reality entirely through Abaddon to see what's next in the sequence."

"Even dath ilan doesn't want to be so Lawful Good - is actively pushing against an extreme of Lawful Good - that people are told to live on somewhere they're unhappy, for somebody else's sake.  If you don't want to live in the Now, you leave the Now to the people who do enjoy living there, and go on to the Future.  Everyone gets told that, nobody is to think that they're trapped, nobody is to think that they need to live for somebody else's sake.  Nobody owes that to anyone.  That's part of the point of cryopreservation."

" - that makes sense but I am having a hard time fathoming the person who'd benefit from hearing it because -

- wouldn't everyone want to exist selfishly, the way you want water when you're thirsty, or air when you're suffocating - why would you have to tell them they shouldn't drink when thirsty just to appease other people -"

"A number of strange people in dath ilan do not, in fact, want to exist, even in the Future, and don't want anything else to exist either, and they started visibly planning to shut the whole thing down, Civilization I mean, by having lots of kids who were like themselves, so they could become a voting majority and vote to exterminate all life, and there was a big fuss, and the Keepers negotiated with their leaders, and their leaders all told the nonleaders that a deal had been struck, and all of them should just go into cryonic suspension and let the Keepers keep the mysterious deal, and that was really really flaming creepy so there was another large fuss, but, being the sort of people they were, they accepted that and went into cryonic suspension regardless of the fuss, so... yeah."

"It actually makes a lot more sense to me, now that I realize that, among the things the Keepers must have told them, was that it was literally impossible for them to stop existing and true-suicide would throw them into a dangerously unknown distribution of universes."

"So that's not even the question.  The question is just - do you have to exist now, or can you just step through time and exist later instead?  And nobody who's not having fun in the now is obligated to stick around in the now."

" - but, you exist less, right, if you spend a bunch of time not existing until the Future. Also I am mentally setting that story aside to be horrified about later since it doesn't seem very helpful right now but I'm horrified."