Re: iPad app for color isolation?
My iPod Touch v4 (oldie but goodie) has an app named "Munsell DG" by an outfit called The Classical Lab. Should work on just about any iThing, but it may be the case that the same app is no longer available. Nevertheless, there are other Munsell-like apps.
The app doesn't pick colors from a photo. Instead, it displays pages of a (digital) Munsell-like color book. Also, I can turn off the colors that are known to be outside the gamut of my device. For best effect, the iPod automatic ambient light adjustment needs to be turned off.
I can go to an art museum, and hold the iPod near a work of art that interests me. Then I can take note of which Munsell-like color is the best match to selected colors in the painting.
At home, I can then mix paint that nearly matches the Munsell-like colors. The trick is that I have to view under the same lighting as was in the art museum. Happens to be Philips tungsten-halogen lighting (I asked at the museum), which I have as a desk lamp. If instead you want to find the colors of outdoor nature, then you'd have to compare using similar outdoor illumination, not indoor lamps.
I say "Munsell-like" colors, because the iPod does not display accurate color. Nevertheless, I am comparing Apples to Apples (pun intended). But it would be wrong to say that I am seeing the true Munsell colors.
Doesn't have to be Munsell. Any library of colors, with different hues, values, and chroma, would do. You can even make a do-it-yourself library of prepared images, and compare colors to your images. Then you don't need an app (this assumes you can prepare images using real image software on a real computer, then upload them to the iThing).