It depends on your skin tone, but the principle is based on the fact that complimentary colors (those on the opposite sides of a color wheel that artists use, like purple and yellow, for instance, or orange and blue, or green and red) placed over each other or next to each other, will blend into a neutral color in the human eye. So you use a concealer that will take your problem area back towards neutral, then a foundation that will give you a consistent presentation of your natural skin tone.
Since beard shadow tends to have a bluish tint, a concealer in the orange range will usually be in order. It doesn't take all that much to do the job, since you are relying on optical blending, not masking. For actual beard coverage, you would switch to a thicker theatrical makeup like dermablend.
Susan's has a great
wiki section on cosmetics that you will want to take advantage of, here:
https://www.susans.org/wiki/CosmeticsP.S. in my profile picture I used optical blending to conceal a shadow. It had been 3 days since I shaved and 8 days since my last electrolysis clearance at that point. By the way, to conceal the redness immediately after an electrolysis session I will use a green toned concealer to remove the red.