Notes from a first colour-season check

I am looking for a practical way to photograph skin tone under stable light. The same wall looks warmer or cooler whenever the camera changes its settings. My first safeguard is a shaded window with no harsh sunlight on my face. The comparison should use the same distance without any smoothing or colour filter. 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 well-organised skin undertone test camera might make it easier to repeat the same conditions next time. 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 would prefer a routine that works with ordinary household materials. I will avoid changing the setup halfway through the small experiment. I am grateful for straightforward advice that avoids overpromising.