Interesting question but it is a more complex issue than you probably imagine at first sight.
Take the same tomatoes; not two different ones but the exact same. Take one photo, and then turn off the lights... and take a second photo. The first image will surely look redder than the second one.
You can do the same with different light situations, a sodium vapor light, fluorescent light, or on a sunny day.
The first basic parameter you need to define is the quality of the light. One quick reference is CRI https://en.wikipedia.org/wiki/Color_rendering_index
The second is exposure. In my example, you can also overexpose the image and you will have no red, but white.
Another parameter is the light environment. Depending on how glossiness a surface is, you could be seeing whiteness on a reflection, or darkness in a shadow.
Ok, let me assume you have all it covered.
What you need to do is prepare is a color-managed photoshoot. You need a color target of good quality.
1. Use one like Xrite's Color Checker. There are other brands, like Datacolor, but I am not sure if it works the same.
Do not use one with too few color targets. That will produce an inaccurate adjustment. Color checker classic has 18 color patches. The one you posted has only 6. It has no information on how to make a curve.
2. Take your photos in RAW.
3. Use either an incident light meter or a gray card to get a proper exposure.
4. Use diffuse light, so you can have better control over the light - shadows fallout. Put the light source at a good distance from the object.
5. Forget GIMP, you need to use a RAW image processor, like Lightroom. Look for some tutorials for this specific step on youtube.
The specific step would include calibration for each photoshoot assuming you are taking the photos in different environments. If you have a controlled light situation, like a photographic studio, you can do it one time for all photoshoots.
6. Now, finally you have something to compare the images. Take a sample on a similar angular area.
7. Define the parameters of your data. Put each RGB value on a separate column or read the HSB values. Probably the "Redder" will be a combination between Saturation and Hue. The most saturated value on the right Hue angle.
Or get one colorimeter or spectrometer: https://www.amazon.com/s?k=colorimeter
Thinking about the project it sounds like a very wet working environment for the color checker, and the color patches are very susceptible to stains with will shift the tone. See if the colorimeter is waterproof or resistant.
Some ideas in order to standardize the project using a camera.
Think of the project as a "forensic lab", and think of real standardized situations, especially if you have several locations.
Use the same brand and model of camera and lens. Use them in manual mode, define the white balance always as sunlight. Shot in RAW.
Get a ring flash of a good reputable brand so you have consistent light temperature and quality. Define a standard power given a standardized distance. I am thinking something like a dentist would use.
Prepare a standardized background. As I am imagining the working environment, it could be some middle gray, directly from the paint bucket, not a prepared one. This way you maximize the viability whenever you need to re-paint or paint a new location. You could also use some defined color of Formica or some other HPL High-pressure laminated brand.
Use a satin finish over a glossy one. Do not use a matte finish. If you can have some strips like the ones you see on a forensic shot, with some color references for exposure, a white target and a black one would be nice.
On one clean location or lab, you can make some standard tests, Optimal distance, test the ring flashes, the variables, measure the neutrality of the background gray, use your color checker, etc.
Capacitation is also a key factor. The people in charge of the quality should have a standardized workflow. For example, draining the humidity of the product to some degree to minimize reflections or something, the angle of the camera, etc.
But I still think a colorimeter will be a better option.