Take a look at the grey areas. Is one darker? Which one? I suppose I’ve already set you up. As you may have counterintuitively guessed by my leading, they are the same shade of grey. You can cover the white and black bars if you need a bit more proof. Granted the tinting from flesh alters this a bit, but the illusion persists. This is known as White's Illusion.

In the first image, notice the right ankle and heel color. It is the same as the knee and thigh. In the second, the grey with black lines is the same as the grey with white lines. Why is this? Oddly enough, this is a grey area itself in perceptual studies. Light and dark perceptions have various components such as expectations of shaded areas (from prior experience) and lateral inhibition. However, this illusion has the opposite effect of lateral inhibition. This illusion has yet to be nailed down. Corney and Lotto (2007) offer a more current and encompassing explanation.