There are several issues here.
when digitally printed (a) coated or (b) uncoated, will be as close as possible?
For a digital print, it is better to use an RGB file.
The result on coated and uncoated can be potentially extremely different, depending on if it is tonner based or inkjet-based.
I would not use a photo as a reference unless you have calibrated your scene+light+camera+lens with a Macbeth chart, which can be as expensive as a Pantone color chart, so let's go to the practical way and specific for your case.
Print your own reference car on the actual paper you need to use, both coated and uncoated.
Here is a color chart that you can use: https://otake.com.mx/Color/RGB-01-Letter-LowRes.png
Go back to the wall and compare the values. The values can be different on both prints, the uncoated will probably need to use lighter values because the print is most likely to be darker.
Now you have the exact value you need to use on your digital file on that specific printer with that specific combination of papers.
If you still need a CMYK value, you can convert the RGB file to CMYK. (Let me see if I can find my chart)
If you need a real absolute value, you could need a Spectro-colorimeter.
There are some other types of color charts cheaper than a Pantone one, called "Color atlas". This will be a good option if you need a CMYK value to be used on a comercial print, like offset. But you need to see what color profiles and specifications they used.
If you still want to go with a photo as a reference, you need to take into account several things.
That different cameras have different flavors, for example, some can make your colors "pop" by saturating them, some will make the skin tones more neutral and the greens greener and blues bluer.
So you need to use a target, normally a Macbeth which is used to calibrate the camera+lens+light+scenario.
But you can do a basic calibration of the scene measuring the white balance of the light.
Here is an explanation of how to do that. https://photo.stackexchange.com/questions/61493/color-issue-studio-images-have-a-pink-hue/61497#61497
You also need to adjust the exposition using either a gray card or an incident light exposimeter.