Carefully, and with colors rather than your eyes alone.
Fair skin shows everything. A flush across the cheeks, veins at the wrist, a yellow cast along the jaw. Most of that sits on the surface, and the undertone is quieter.
So pink-looking skin doesn’t prove a cool undertone. Ivory-looking skin doesn’t prove a warm one. Redness is a surface matter and can come with either. That’s where the old shortcut, “pale means cool,” falls apart.
Use the colors instead. Face a window in daylight, bare-faced, with your hair tied back. Hold Peach under your chin, then Powder Pink. This pair suits light faces, so treat it as one lean, not an answer.
If Peach smooths your skin, you may lean warm. If Powder Pink does, you may lean cool. If neither clearly wins, you may be neutral, which is a real reading and not a failed test.
Then run a second pair that fits your depth, such as Coral against Raspberry. If the same temperature wins both rounds, you have a direction.
My advice: skip the bathroom bulb. A yellow bulb warms everything, your skin included, and it will push you toward warm every time.
If your results disagree, six undertone tests and where each one fails can help you sort out which signal to trust. For the vocabulary itself, what warm, cool, neutral and olive look like is a good companion.