Question about checking an undertone at home

I would like to take undertone photos without letting my phone decide the colours. The same wall looks warmer or cooler whenever the camera changes its settings. I want to begin in even daylight instead of mixing several light sources. I will keep the same camera distance and avoid filters, portrait mode, and flash. A grey card or plain white paper may give me a useful reference in each frame. I also plan to repeat the setup on another day before drawing any conclusion. A simple skin undertone test camera could help me remember the steps while I keep the final judgement open. I do not expect a digital image to answer every undertone question. I will compare the images side by side instead of relying on what I remember. How do you keep your phone from changing the comparison halfway through? I am hoping to keep the process accessible and repeatable. Keeping the original files feels important when automatic processing can be so persuasive. Thank you for sharing any small details that made your comparison clearer.