what undertone means
Undertone is the word makeup people use for the hue behind a shade: cool, warm or neutral. It helps when you're choosing between two foundations. It isn't a test, and I can't tell yours from here.
1. cool, warm, neutral in plain words
Cool: shades that lean pink, rosy or bluish.
Warm: shades that lean yellow, peach or golden.
Neutral: shades in between, a mix of pink and yellow.
These describe the shade in the bottle. How it looks on you is what you check next.
2. comparing two shades in person
Stand by a window in indirect daylight, not direct sun. Warm bulbs can make a shade look more yellow, and colored lights change it too.
Swatch two: put a stripe of each shade side by side along your jaw, toward your neck.
Give it a few minutes: then look again in the same light.
Pick the quiet one: I go for the shade I can't see at the edge.
what this guide can't promise
I can't tell your undertone from a photo, and a guide can't either. Lighting and screens shift color, so the checks here are hints, not a verdict. If two shades both look right in daylight, pick the one you like.