Warm vs. cool undertones, explained
Your surface tone is what you see at a glance — fair, medium, deep — and it changes with a tan. Your undertone sits underneath and stays put. Warm undertones lean golden or peachy, cool undertones lean pink or blue, and neutral sits in the middle.
Undertone decides which 'side' of the color wheel tends to flatter you. Warm folks usually shine in yellow-based colors; cool folks in blue-based ones.