Like many people at this time of year my neck is lighter than my face because the sun doesn't really hit my neck. Rather than choose a lighter foundation shade to match my neck (which tends to make me look ill and needs significant bronzing up) or use a darker shade which might result in a tide mark along my jawline unless I blend my foundation all the way down my neck, I just put a lighter shade of foundation along my jawline. I then use the shade that matches the rest of my face on my cheeks, nose, forehead. I can usually blend the two areas together seamlessly. It also has the effect of sculpting the jaw slightly, partiularly if you apply a contour shade underneath the jawline. I don't usually have to buy a lighter shade of foundation, most of us have a summer and winter shade of foundation, or I use samples of foundation that were too pale for me.