Paint Matching for Rookies

Q....Is it easier to match paint that's still in the can or on the car?
I'm a bit skeptical on those color cameras.
I tried (my first time) painting single stage on my '69 front clip recently. It didn't go as well as I had hoped.
Since my car was not drivable, I pulled the trunk lid and brought it to the paint shop to get the color I needed, allegedly B5 per their color match camera. Well, it came out too light. So back to the paint shop we go. The color match camera was tried again. It determines there's a hint of green so the Tech re-mixes it.
Hmmmm..........came out a touch dark this time..........but it is blue, and I still have more BW to do down the sides and I'll need more paint, hence the question.
TIA

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