Skin color change?

My dog (5-6yrs old) had three lumps removed from his skin. One here at this bald spot pictured and two on his back. When he came home, the freshly shaved spots had pink skin underneath. As weeks have gone by, the skin that was shaved has become darkly spotted. In one spot on the back, it’s become completely dark skin. No discomfort associated with any shaved region. My husband and I adopted him 5mo ago and don’t know his back ground. We believe he’s a shepherd/border collie mix? Anyways, does anyone know why this change in skin color would occur?

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