I have been attempting a simple undertone check with my phone, but the result changes every time I move rooms. Different bathroom mirrors seem to change the apparent balance of my skin much more than I expected. To make the test less random, I will take several photos near the same window on separate days. I will keep makeup, filters, and automatic colour effects out of the process. Before deciding from one photo I would compare a few neutral fabrics beside my face. Navy, cool grey, berry, and gentle blue would give me more than one clue to consider. It may help to write down whether each shade makes shadows, redness, or unevenness more noticeable. I want to look at the images together later, not one at a time. I found this overview useful while setting up the comparison:
skin undertone test camera What would you standardise first before comparing the photos?