I seem to recall that the pen bodies of Preppies are PMMA. I just checked, and the barrel of the preppy in front of me is alcohol-safe. The collector and feed might not be, but I hadn't heard that they were not before.
I will note that, while you might find the color dull, Noodler's La Couleur Royale is amazingly easy to clean out, right up there with Waterman Blue and Montblanc Royal Blue (neither of which I listed before because they are neither vibrant nor saturated). La Couleur Royale is highly saturated but not vibrant at all.
I would say waterman and montblanc have the easiest inks to flush out, at least from what I have experienced.
My easiest to clean inks include OMAS sepia, Pelikan Blue/Black (NA in NA), MB Royal Blue, Waterman Florida & Mysterious Blue.
Regarding the Preppies and using rubbing alcohol, when I used it once on a Preppy the plastic turned dullish white inside the section, but otherwise the pen still works.
I had Waterman Serenity Blue, and that ink was amazing in that it cleaned out very easily and looked good on even the cheapest paper, but it didn't have any water resistance at all and I didn't like the color. I've since tried out Mysterious Blue and Harmonious Green, but none have really grabbed my attention.
The Corn Poppy Red looks great, I think I might check that one out. Le Couleur Royale has me intrigued, might get a sample of it.