It sounds like a very cool idea, but I'm concerned that, even with just one coat, the Tarrago coatings (sorry, but as you may have already read in other Tarrago threads [LOL! :shame: ] I don't think of most of the Tarrago 'dyes' as dyes, as such - as they just coat the leather, rather than sinking in, as a true dye would and also, I believe all metallic dyes [not just Tarrago] are just coatings, anyway) might be too thick to let the original colour show through evenly; especially as they're specifically designed to cover any colour completely (rather than just adding a subtle sheen to the original colour).
If you are going to do it, I think trying it out first on something expendable is a great idea. However (and I'm no leather expert, but), I assume that different leathers with different finishes (or no finish) will, presumably, take the paint quite differently? So even if it works well on a cheap bag, that already has coated leather, it might not work so well on one with a differently finished, or unfinished, leather (or maybe it might work better, I don't know?)?
Also, as you've read some threads, I take it you've probably already read the tales of peeling and so on, with Tarrago, haven't you?
Anyway, if it did work on a cheap bag and I was going to risk it on one of mine, I'd, personally, go for the navy - as I think, being much darker, it would be less likely to streak, or look patchy (assuming you are intending to do only the one coat, with the idea of letting the black show through) and I also think it would be a cooler, more subtle finish over black than platinum, anyway.
As I say, I think it will, almost certainly, be too thick to let the black show through, though - I think what you really need is some sort of translucent metallic/pearly finishing product, rather than an opaque metallic dye/coating.
I don't know if anything like that exists?
Anyway, good luck!