For years I've been mistakenly "accused" of having a pink undertone, because my face has quite a lot of redness. A bit over a year ago, I realized that my skin was in fact neutral leaning on yellow and some of my favorite foundations had the wrong undertone. Always match your foundation with your chest and neck.
To understand how to neutralize something, you need to know about color theory. To opposite colors on the color wheel cancel each other. Therefor, to remove the red pigment from a foundation, you need to neutralize it with green. If you need your foundation to be yellow, you need to first use some green adjuster and then add some yellow. Various professional brands offer color adjusters that you can mix with your foundation.
Because I'm not a professional make up artist and that I don't have that many pink foundation left, I decided to neutralize my foundation with a green primer and then finish it with a yellow finishing powder. If you have strong yellow undertone though, buying adjusters might be a good investment. Women with fair skin tend to have less strong undertones.
The too pink foundation (It's a light coverage foundation, but trust me it's pink!)
adjusted version on the left, untouched on the right
As you can see, mixing the foundation with green turns it into a neutral shade.