Separating camera color from actual undertone

[GeorgePeS]

I've been trying to work out my undertone from phone photos, but the answer seems to change with the room. Under warm lamps my skin looks golden; near a window it looks much cooler. I would like a method I can repeat instead of trusting one flattering photo. I can take a fresh set of photos in indirect daylight, with the same camera and no beauty filter. That should at least keep the lighting more consistent. Does putting a plain white card in the frame help spot white balance shifts? I'm uncertain whether makeup would make the comparison less reliable. For a separate appearance check, I found color analysis and thought it might help me frame the comparison without treating a screen result as proof. I would rather compare several signs than decide from a single selfie. What would you keep constant when repeating an undertone check? I could hold different colored clothes near my face to see what changes. I can repeat the check a week later to see whether the impression holds. I'd appreciate practical steps, especially ways to catch a misleading camera result before buying anything based on it.