Ideal PNP Quality?

What I would look for in a PNP game (or any game really) as far as aesthetics go--apart from appealing to my own personal taste and quirks--primarily boils down to coherency and thoughtfulness. I'm not that concerned about "polish" in the sense of fanciness or "high production value". I do like it when I can tell that that effort invested in crafting the look and feel of the game regardless of how rough or lo-fi it is (sometimes I find those traits themselves kind of charming). As far as consistency/coherency goes, it's mainly a matter of not having clashing design elements that are distracting. Public domain or clip are is fine so long as the art meshes together well as much as possible. I do think the two-phase PNP development sounds like a good idea as well.

That is a correct interpretation.

Here is a link to one of my first prototypes of the game:

Compared with a more final version:

There is a difference - but the B&W still illustrate the basic layout of the fully rendered version (with artwork).

I prefer to stay away from clipart - because it looks very amateurish. If you have clean, crisp B&W layouts, I think this could be used until you say reach a funding goal. BTW I can't fully afford my artwork - so what I did was ADVANCE funds for 10 pieces of artwork. I will then use those as SAMPLES of what we can do - and how the game will finally look like.

But I don't think B&W prototypes don't work. Just depends how much effort was put into them!

Note: BTW I think the B&W version allow for a CLEARER delivery of LAYOUT of the card itself. There are no distractions of say "Pretty Artwork" just a functional card...

Most of my prototypes use basic clipart since I have very limited artistic ability. And, I usually provide a B&W version for people that don't want to use a lot of ink.

This is for a kids game I am working on. The raindrops on each flower card indicate how many blue glass beads the player can drop.

The "Colored In" version looks like its been colored in by crayons (using a photoshop brush) and the black & (off)white "Coloring Book" version is self explanatory.

I still think a playtester could enjoy the game even without the color.

What market are you targeting? Younger players and professionals are more likely to reference the rules from a phone or tablet, printing only what is necessary, which makes color artwork more important. Students generally have ready access to high quality printers, so the cost of ink isn't much of a concern for them. Veteran gamers, which is to say older players who've stuck with the hobby for at least a couple decades, are less likely to worry about polish.

That said, 'lacking polish' should only be an illusion, like when a couple hours are invested into giving a model 'bedhead' for a photo shoot. The simpler the illustrations are, the more important the design elements become. There are a lot of great games that will never get the play they deserve because the presentation is so amateurish.