If the female is indeed fairly black on the top of her head and you purchased her, there is a fair chance that she was raising in a pond during some part of her life. If that is the case she might be infected with larval digenetic flukes. The digenetic thing means that they have several hosts in their species life cycle - including snails from the wild and fish eating birds. The cycle will stop with her (unless you have a pet seagull), is usually harmless, but can't really be treated. I hope that is what it is and all you have to worry about is the treatable ich. (This from the Tropical Fishlopedia by Bailey and Burgess)

That "sort-of iridescent purple colour under the right light, and just gray-white at other times" sounds disconcertingly like columnaris, which is a fast acting bacterial problem. Please do a Google image search for what it looks like (or check the GL articles alluded to below). If you decide it is the very contagious and fast-killing columnaris, please consider the suggestions below.

My GL search also suggests you might take a look at

disease again, columnaris    
Health and Medicine
By Talonstorm [Edit User]
from the Tina department, Section Ask Guppylog
Posted on Tue Aug 31st, 2004 at 01:46:19 PST

And

Female guppies in the Northeast,,,    
By guppygirl
from the guppygirl department, Section News
Posted on Tue Jan 13th, 2004 at 08:55:46 PST

Google also suggested

White belt around females and male guppies    
Health and Medicine
By krickkett273
from the Krickkett273 department, Section Ask Guppylog
Posted on Thu Jul 22nd, 2004 at 06:45:36 PST

(Some ID links are suggested)

shedding    
By dollfin , Section Diaries
Posted on Thu Mar 25th, 2004 at 12:03:17 PST
MY FEMALE GUPPY HAS A WHITE LOOKING SUBSTANCE NEAR THE END OF HER BODY. IT ALMOST LOOKS LIKE SHE IS SHEDDING SKIN. IS THAT POSSIBLE? WHAT SEEMS TO BE THE PROBLEM AND WHAT CAN I DO TO FIX IT?

HELP Couminaris!    
Health and Medicine
By cloe [Edit User], Section Ask Guppylog
Posted on Wed Sep 17th, 2003 at 11:49:18 PST

Lost half its color...OVERNIGHT!    
Care Tips
By ashleythefish
from the Ashley the fish department, Section Ask Guppylog
Posted on Wed Apr 21st, 2004 at 16:30:45 PST

I'm sure we both wish that those neon tetras had been put in a quarantine tank. That really complicates things. If you can hold off the ich long enough to deal with the columnaris - if that is what the problem is - go for it.

Your Coppersafe/ Maracyn 2 combo is an interesting idea if the Maracyn 2 is an antibiotic and compatible with Coppersafe. What do they say on the bottle?  Does Mardel Labs have a web site where they might address the compatibility of medicines? Theoretically the Coppersafe might hold off (if not eradicate) the ich while the Maracyn 2 may treat the columnaris or other bacterial infection (fin rot).

I think you were correct in suggesting that the flashing was a response to ich. I appreciate your sensitivity to not wanting to mix meds. I'm assuming that you are making significant partial water changes between meds (or returning activated carbon to the filter to take the first medicines out.) Continue the partial water changes between medications. If you have another tank, be careful not to infect it with that siphon tube, the bucket or your hands.

It would be nice if you went to a shop, they had Metronidazole and the package said that one could use methylene blue with it. Or they had Phenoxyethanol which could be used as a bath for the columnaris and then you could treat the ich. (Always use the Phonoxyethanol as per the manufacturer's instructions so the fish are not over-dosed and "put to sleep" permanently.)

A historical note: Coppersafe, chelated copper, is a lot safer to use than the copper scouring pad a pet shop owner used to leave in his Anabandid tanks during velvet season or the pennies which old-time aquarists would toss in a suspect tank. However copper in too great a quantity can still be toxic to the fish. Even at medicinal levels it will take out any shrimp you are using for clean-up detail and possibly the snails. (Not a good thing if you have a mystery snail - see Miskairal's observations on a dead snail.) Copper is used to treat some skin dwelling protozoans (maybe including ich and velvet?) and monogenetic flukes (parasitic worms with only one host in their life cycle). It is also used against velvet, though I would recommend dimming the lights, changing the water, adding a teaspoon of salt for non-salt sensitive fishes and medicating with on of the acriflavin applications instead.

I was surprised to find a recipe for treating ich with a long-term salt bath in Burgess, Bailey and Excell's A-Z of Tropical Fish Diseases and Health problems. This is only to be used with salt tolerant fishes ' which would include guppies and most livebearers. (Corys might not be so tolerant.) Two grams per liter is their suggestion.

You may be more able to  "get away" with the use of an antibiotic and salt. Wouldn't it be nice if your LFS had a strong antibiotic/ methylene blue combo? ;)

An American gallon = approximately 3.79 liter.  An ounce = about 28.38 grams. So they are suggesting roughly 5.2 grams of salt per gallon. It might be easier to add an ounce of salt per six gallons of water. Since the water content of most "ten gallon" tanks is closer to 8.6 or 8.7 gallon and gravel is included in many tanks, maybe 1.2 ounces of salt could be added to a ten gallon tank. If you don't keep that ten-gallon tank all that full or have a lot of decorations, just add an ounce of salt to the tank along with your medication. (My math is always suspect, please feel free to correct it.)

Good luck. I apologize for the rambling answer. ID your problem best as you can. Then treat the most threatening disease first. Double-dip medication only if it seems safe.