Answered[ Edited ]
I can't tell you how many times I have been "matched" incorrectly -- part of it has to do with artificial lighting, but the other part has to do with where on the face the foundation is tested.
The key is: Be *sure* to test the foundation not only on your jawline, but also your neck. As Jill pointed out, matching the neck is key because your face may not match match otherwise. From a personal standpoint, I have a lot of natural redness in my jawline and cheeks -- so, as a result, it makes no sense to swatch foundation in this area, because I always wind up with a match that is 1-2 shades too dark. When I match my neck, though, my face looks entirely uniform -- it is a much better indicator.
The other key is to go outside in natural lighting and be sure that it actually matches. I have had foundation shades look decent in the department store / Sephora, only to leave and realize -- eek, my face is orange.
SInce everyone's skintone is different, I would suggest going into your local Sephora store and ask for assistance from one of the Color Cast members. They will ask you questions about what exactly you are looking for and will find out about your skin and recommend a foundation formula that will work for you.
Then they will color match you and ensure that you are happy with the match. I like to match my foundation shade to my face and also apply a dot on my décolleté to ensure it will look consistent with my body as well.
You can also have the foundation applied all over your face to ensure that you like the way that it looks.
They are trained in all brands and really know the products well. I hope you find something that you love and know that you can also ask for samples of the foundation to try out if you are unsure.
Best of luck,
Sephora associates are definitely super friendly...I actually went into a Sephora store a few days ago and although the associate was friendly, she wasn't too helpful with picking out the correct foundation shade. I knew I wanted to purchase a Sephora Mattifying Compact Foundation to help with my extremely oily skin so I told her what brand I wanted and she showed me the selection. She ended up picking a shade that was too dark for me and was not the correct undertone, as my undertones are warm and she picked out a cool shade (R). I asked her if she thought it was too dark and she said no. I told her I felt that my skin was more suited to a warm (D) shade. She still picked out a shade that was too dark (D35) so I just thanked her for her time and went browsing on my own. What helped me to pick the correct shade was holding different compacts near my chest area because since the skin there is makeup-free and my bare face is the same color as my chest. I ended up choosing a perfect, flawless match and am so pleased. I'm sure there are sales associates who are better than others at helping you choose the right shade of foundation for your skin so you should be fine. I would definitely go in prepared to tell them what type of undertones you have as it can be hard to tell if you already have makeup on...especially makeup that does not compliment your skin tone.
Have you tried Fusion Beauty TM's? Awesome. They look darker in tube than on skin. I use caramel...it's perfect on my light-med skin. Also love Josie Maran Matchmaker (light-med). I've tried everything under the sun and these are the ones that worked for me.