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I am a dentist   and use photography clinically to record disease, to plan treatment and to show people before and after photographs. I would also like to start using some of my portfolio to educate other patients and to promote my work. Getting reproducible colours between photos is extremely important, especially if I am doing tooth whitening for a client or taking a shade for my laboratory technician to make a crown in a matching colour!

I have not used any photo editing software before so am a complete new learner. I need to decide which program to buy and my editing needs are very basic: cropping, correcting white balance, and mirroring will be 95% of what I will do. I do know from courses that Photoshop has a function to use 17% grey card in a photograph to set the white balance for a whole collection of photos. Does Gimp have this function to use grey card as well and if so how do I do it? This will really help me decide which software to buy.

Thanks very much!

There is the canonical script for this: Luca De Alfaro's whitebalance. It requires a small update to work in Gimp 2.10. You sample a color that should be neutral in the picture (ideally a grey card, otherwise a piece of print paper or a white fridge will do) and it tweaks the color balance to make that color grey. There is a similar feature in more advanced/specialized photo editors (RawTherapee, Darktable,..., that will also process JPEGs if you don't want to take the trouble to "shoot raw").

However, that means R=G=B in the image data and nothing else. What you see will depend on the color calibration of your screen, and on the calibration of the screen of your technician, all this under the control of the image color profile if there is one. Not so easy.

IMHO doing a full color calibration each time is going to be difficult because putting a grey card in the patient's mouth isn't going to be easy (while being completely necessary, because the GC has to receive the very same light as the picture). It would be easier to take the picture with a reasonably constant/reproducible lighting,, find its color temperature (possibly by shooting a grey card), and then use this color temperature to adjust the color balance of further shots. It could also be possible to set this color temperature in the camera settings, so you would get color-neutral pictures out of the box.

Warning, Canon minion speaking: Canon has recently issued a pair of macro lenses (one for mirror-less,cameras, and one for DSLRs) that have a built-in ring light around the lens (in addition to the hybrid stabilization specially designed for macrophotography). They are pretty much what I would have dreamed to have to shoot inside the mouth of people with a controlled lighting, especially considering their low price.