[Pazuzu]

I work in optics, and have background in getting digital cameras to properly recreate color and hue correctly...

Definitely a built in white balance problem with the camera. You need to find a way to output RAW if you're going to compare at that level of detail.

Try this...3 pictures, one after the other, with the camera on a tripod. Recreate your wheel image. First, take an image with both wheels, then take away the small piece, then take away both and only image the ground. The three SHOULD look identical as you flip back and forth, but they won't. The camera is looking at the histogram of each color channel and finding the median (or some other internally decided value), and setting that to be "white". The full wheel is getting more sunlight and a brighter spot, so it's probably deciding the color balance. The camera might even have zone balancing, so it's setting each differently.

With RAW, you should be able to go between the 3 images and see the exact same colors/levels/hues between each, which is when you're looking at the actual raw pixel level data. THAT is what you need. The files will be huge (on the order of a Gb each).

Looks like the Nikon D7200 has a "manual preset" white balance, use that, the manual says settings from 1-6, see what "1" and "6" do on a test image.
Also, set output to NEF (RAW) and 14 bit depth, instead of jpeg. JPEG files have their own set of internal calibrations that change the picture even more...
ALSO, see if you can get an image using manual exposure and aperture settings, the auto settings might change the values between shots as ambient light changes.

In other words, take that expensive hi-tech camera and make it stupid, all of the bells and whistles are modifying your image behind your back, and you're critical enough that you can see it.

When you work on Photoshop, keep it in some high-bit format, like TIFF (or RAW, if it can keep the image that way). RAW file format is 3 numbers per pixel giving the RGB amplitude and nothing else. It's the actual data that the CMOS detector picked up.