I swear, it's the million dollar question...'What's my undertone?!' First things first, though...let's break down the difference between under tone & skin tone, because knowing both is the key to matching foundations, concealers, powders, AND knowing what colors look good against your skin!
Your skin tone--put in my own terms, FYI--is the level of your skin color on a spectrum from light to dark. Your skin tone typically changes throughout the year with increased or decreased sun exposure. Under tone on the other hand, is an inherent color that comes up from beneath your skin, and is largely due to a visual combination of blood vessels, melanin pigments, etc. Your under tone is something you are born with and it won't change, so once you know it--you know it!
The thing is, undertone can be hard to detect with the naked eye. Especially in your face, which can have a lot of 'surface tones' (redness, sallowness, dark spots etc) clouding your judgement. So how can we determine our undertone once and for all??
If you go to most websites that discuss undertone, they leave you a trail of clues to follow...like whether you look better in silver or gold, whether your veins show up as a certain color, etc. Feel free to check some of these out and gather up some clues. Then, head to a big beauty store like Ulta or Sephora and do something I invented called "the comparison test".
There are plenty of brands now that have their foundations divided by undertones into cool, warm, and neutral categories. One of my personal favs for this test is Cover Fx. Whatever brand you choose, find a selection of at least 5 foundations in each undertone category, and do swatches of them all lined up side by side on your inner arm. What I mean is, swatch 5 or more 'warm' undertone foundations next to each other. Then swatch 5+ 'cool' undertone foundations. Then 5+ neutral undertone foundations. When you start to see groups of these tones not only together against your skin, but also side by side, I find it WAY easier to see which category seems as if it could be a part of your skin. It's ok if none of the foundations match your skin tone...it's all about the overall tones that jump out at you.
Now, I know this is not an exact science, and different brands have different pigments in their foundations that can look off-color...but I'm telling you, for a DIY solution, this comparison will help you see the color in your skin SO much easier than judging on your own. In fact...when in doubt about whether ANY makeup color will look good on you —whether it's a lipstick, eyeshadow, etc.—you can always tell by lining up several options on your inner arm which ones look awful, and which might not be so bad. Comparison is your friend!