The Skin's Undertones:

Skin undertones are the colors that lie beneath the skin. Undertones are the colors that are cast like a shadow beneath your skin color. People of color undertones are usually red or yellow.The shadow has a distinct color which never changes. While the tone of your skin color can change very easily your undertone never changes.
You'll either have cool, warm, or natural tones:

*Cool undertones for darker complexions are usually bluish to violet in color.

*Natural undertones usually consist of olive (green/yellowish) or yellow colors but is very hard to distinguish.

*Warm undertones on average is the most recognized among people of color. Warm undertones consists of yellow, orange, red and olive with each one growing in intensity and depth of their color spectrum.

The way to determine your undertone can be tricky, but the best way is to observe the color of your veins (possibly near your wrists):

1. If your veins appear to be bluish in color then you fall into the cool category. This also means your undertone is on the blue or pink.
2. If your veins appear green then you fall into the warm category. This usually means your undertone is on the yellow/peach or olive.
3. With natural undertones the vein color is less obvious in color and can not be distinctly recognized.

Another of way of finding out is going to the mall and have a makeup specialist tell you, this is what I had did to double check but I already knew my undertones were yellow.
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There are Fitzpatrick Charts and other chart types to help determine your skin shade and tone:
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also:
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Fitzpatrick Classification Scale