Line of Demarcation
Nothing lets someone know you don't know what you are doing like the 'ol line of demarcation. The line of demarcation is the area in which there is a difference in color between the foundation on the face and the actual color of the face/neck. This effect is intensified when a foundation oxidizes or is the wrong shade to begin with.
There are several ways to prevent this issue. You can bring foundation down your neck so that it's all one shade but the problem is that the foundation doesn't match which is just substituting one problem for another. Not to mention, how are you supposed to hug people or wear clothes without getting foundation all over them and your clothing? What I do is wear foundation that is low coverage so that even if it isn't an exact match, it's sheer enough that it doesn't alter my actual skin color by much. It just evens out my skin.
Now I am aware that not everyone is comfortable wearing sheer foundations. Some people just prefer full coverage foundation or have a skin/complexion issue that they are trying to correct or cover. In that case, match your foundation to your neck color and not your face. I think it looks more natural to wear a foundation that matches your face even if it's lighter than your body because that's what's probably already the case. Most people have lighter faces than bodies due to wearing sunscreen. Aaaand, then you can apply blush, bronzer and powder which will darken your complexion anyway which will then make it closer to the color of your neck and décolletage. So you can wear a sheer foundation or tinted moisturizer, match foundation to your neck, or wear foundation that matches your face and darken it with other products. It's amazing what makeup can do to balance out other makeup products. Of course, always look at your makeup in natural light first. It's easy to think foundation matches in the wrong light. You have to go outside and actually look at your face and neck to check that everything looks seamless. Also, demarcation lines can be anywhere on the face between different products. For example, blush that's applied as a stripe down the face. I always take another brush and buff out other products to help it blend into the surrounding skin. All makeup should be blended unless you are going for a specific look or something graphic.