|In natural sunlight, look at your inner wrist to see if your veins look green, blue, or purple. If green, you have yellow in your skin and therefore you have a warm undertone; if blue, you're probably neutral; if purple, you're cool.
Another way is to hold up a white piece of paper or cloth and compare it against your skin tone.
If you're a typical fair-skinned Irish person with red hair and freckles, you're most likely cool. Stand next to this person to see what your skin color looks like.
If you're an Asian or Mediterranean with an olive skin tone, you're considered warm. Golden undertone is also warm, but there are all shorts of varying warm shades - you can be olive (more green), golden, or peachy warm.
If you just can't tell what color you are, then start with a neutral shade. Most caucasians fall into some type of neutral to slightly warm shade.
Also, to get the most natural look, match your makeup color to your neck so that it blends in to the rest of your upper body.
Foundation Shade Chart