Just got a note from the author of Scribbles ( wave :-), and turns out that the one feature I was missing - the "eyedropper", the tool to match an existing color - was there all along; there's a small set of crosshairs next to the color selection button/icon, and if you click on those, you get a magnifying glass color selector.
Here I was, thinking "ok, there's subtlety to doing this right and I can see there being reasons to not get to it" - and those of you who have worked with me before know that I don't get to give user interface advice - but no, it's actually right there. I think the only reason I missed it was that it's the only small UI element in the whole app; the author clearly appreciates Fitts' Law... but there isn't any wasted decoration in the entire app, so I should have actually tried tapping it to see what it did :-)
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