How do you make a colour analysis quiz useful rather than overwhelming

[AmeliaWape]

I am trying to understand my undertone before ordering makeup samples, but my phone camera keeps changing the result. Near an open window my skin looks fairly neutral, yet the same photo taken later can look noticeably warmer. I removed filters and I am using the rear camera, although automatic white balance still seems to interfere. A white sheet beside my face helps a little, but I cannot tell whether I am comparing colour or simply reacting to contrast. I use a plain wall behind me to reduce differences that have nothing to do with my skin. I want to keep the test simple so I can repeat it on another day instead of chasing a perfect single image. I plan to compare photos without editing rather than relying on a memory of how the first attempt looked. I found a skin undertone test camera guide while collecting ideas for a home comparison and plan to record the time, light, and the colours placed near my face. If you use a phone for this which conditions should stay fixed before I compare the photos?