"Use it to try cleaning quartz, I'll talk to him."

...huh.  Somehow Ione wasn't expecting that much of a reward, that she'd get to be the first to use the spectography station on quartz.

Well, she'll go right out and try it, then!

(But is not, in fact, the best at Prestidigitation chemistry, as yet.)

"Hey Keltham, is there a procedure for if I have a question that is mildly time sensitive, but not important enough to disturb your concentration over, and I don't know how to tell if asking it will disturb your concentration or not."

"Meta-question too complicated, just ask the object one and I'll ignore it if it's also too complicated."

"Okay. We added a Security casting Major Image, got it working. Change any plans?"

"Assuming I'm still needed to go on concentrating for whatever reason, like my light sources are better, go have everybody else carry out the original plan.  Otherwise, tell me I can stop."

"Ideally you go on concentrating on the stations but not the prisms."

There's no way in the Abyss he could've done that a week ago, but Keltham will try.

She'll focus very diligently on trying to match her illusion more closely to his now that there's the spectrometers to make it clearer what the differences are; once she gets that down they won't need his.

Keltham's white light going through a Security-cast prism creates a rainbow that's - smoother? more evenly lighted in every color? - than Carissa's white light that looks exactly like Keltham's white light to the naked eye.

Weird. She will - try to imagine her light different and see if any of the differences produce different rainbows.

Imagining her light looking the same white color tends to produce the same uneven-looking rainbow.

If she wants to try imagining her light looking different colors, she can produce different rainbows.

What a bizarre constraint on the spell. What if she imagines her light is sunlight. What if she imagines it's firelight. What if she imagines it's Dancing Lights.

The rainbows of different lights sure are different-looking!

Sunlight's got a lot of green in it and less blue than Keltham's source.  Firelight is mostly orange and red as you'd expect but also has more green in it than you'd think, not nearly as much as sunlight though.  Illusionary imaginary Dancing Lights cast a narrow light band that looks like basically the same color as Dancing Lights themselves.

"What are you imagining as the light source to get a full rainbow."

"Light source that's a fairly even mixture of frequencies as produced by am afraid to think too much about it right now but I've seen it so I know what it looks like."

"If you can't get it by seeing mine, try adding and subtracting light sources and colors from each other maybe."

Fire and the sun and Dancing Lights.

To the naked eye, if she tries combining the illusions into one, it just looks like sunlight again.  Maybe if she imagined the sun part to be less bright?

Spectrographically, you can't really see the fire-rainbow part inside the sun-rainbow, which already had plenty of red and orange.  But - yep, she can see the sharp line of the Dancing Lights in there!

She...actually can't readily think of very many other light sources. Mostly light comes from fire, the sun, or her cantrips. Lantern archon? Light cantrip, if it's different from dancing lights?

The illusionary lantern archon's rainbow looks like dimmer sunlight, an illusionary Light cantrip is also a narrow peak of light the illusionary cantrip's color.

Okay, she predicts she just actually isn't going to be able to imitate Keltham's light source until he tells her what it is. Though there's no reason not to continue to try various combinations which she strongly expects not to get anywhere: sunlight with blue dancing lights to make up for the insufficient blue? No; dancing lights are too narrow. Overlapping a hundred hypothetical dancing lights at different points on the rainbow? She can't hold them all in her head.

Don't shut down the party just yet?  Pilar is making much faster progress on Prestidigitating things to burn in different colors, now that she's getting rapid feedback on the subtle results of anything she tries, instead of waiting for the first result visible to the naked eye.

Sorry, Keltham is actually going to lose concentration about now, and then fall over with a melodramatic ceremonial thud for several minutes before he tries talking to anyone.

Carissa will wait until he's recovered to ask him questions about light.

Right, uh, so, sunlight's going to have, like, hydrogen lines in there, and produces more green than blue to begin with.  But the larger problem is that sunlight is missing a bunch of blue from how the air scatters the sun's blue light all over to form the daytime sky.

Firelight, plus metal heated white-hot, plus, ideally, metal heated blue-hot, superposed, would plausibly work for the light source?  If each illusionary component had realistic rainbows underneath the illusion?  He's surprised she couldn't just copy Keltham's light directly.  Maybe there's something here about, if you've seen a real thing, you can make an illusion of that thing.  But if you've only seen an illusion, you can only make an illusion of how a thing looks to you, because that's what an illusion is... maybe... if that made any sense, or rather, if it made the correct amount of nonsense for magic.

Keltham has a feeling regardless that Carissa might be well-served by just messing around with this until she can control the light spectrum of illusionary light period.  Get her magical control to where she's controlling the rainbow underneath the illusion, not controlling how the illusion looks to her eyes.  That will plausibly be the equivalent of a Spellcraft exercise for this, and might be preliminary towards being able to Prestidigitate chemical properties.

Though he doesn't actually know the two manipulations would go through a common route, he's just guessing.