With respect, you cannot trust a phone's camera to render colours 'well enough'. Even with a clever app and test-cards, using RAW files etc. Beyond the obvious issues, there's also the way cameras may 'see' some infra-red, or be skewed  by fluorescence from UV.

Which is where the hand-held 'contact' sensor comes in...

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Another 'A-Nik-Dote': When a long strip of our dark-red, textured hall carpet 'went missing' during re-decoration, we had problems finding even an approximate match. My wife and I, together or independently, must have visited a dozen 'interior' shops to study their swatch-stacks, compare the 'postage stamp' sample we could spare. Few came close, none came close enough.

Then, one night, driving home in the rain, I had to take a diversion due an accident. There, glimpsed in front window of a little 'interiors' shop on other side of busy street, was our match. Took me a week to get back there, an hour to go through their mountain of swatches which, of course, had moved around. Found it, noted the reference, went back at weekend with my disbelieving wife. Her double-take at the perfect match of my 'spot' was wondrous...

Funny part is that my wife could glance at something, rattle off its precise shade or hue. eg 'This blouse is Calabrian Umber, isn't it ??'

Where-as I have trouble naming colours beyond 'Ish': Green-ish ? Sorta greenish-blue ? Or bluish-green ? Pale red-amber ?