We’ve all been there: You swipe on a foundation sample that’s seemingly spot-on, only to find you’ve got a less-than-flattering streak running down your neck. The culprit of this makeup faux pas? Blame it on choosing the incorrect undertone, or the amount of cool or warm tones. To find your perfect match, Olivier recommends doing what she likes to call "the vein test" (strange, we know, but hear us out). First, check out your wrists. If your veins appear blue or purple, you probably fall in the cool range, meaning your skin gives off pink, red, or bluish tones. If your veins skew more green, shoot for warm shades that’ll match the yellow or golden undertones in your skin. A mix of both? You land right in the middle in the neutral category. When choosing a foundation, look for a line like Too Faced Born This Way Foundation that clearly lists out the exact undertone in every shade. This buildable, coconut-water-infused option gives off a natural finish. Plus, the formula is enriched with alpine rose, which brightens skin for a covetable glow.