What is the difference between a blackhead and a whitehead?

Color: Certainly, blackhead looks black and whitehead look white, but it's not that simple. Blackhead and white head both are mild/early forms of pimple/acne. Blackhead devolops when the content of pimples, mostly, sebum, expose to air and oxidized. It turns to dark brown or "black" color. Whitehead is when sebum enclosed by epidermis(i.e., not exposing to air) and showing yellow or "white" content.

White and black head: White heads are skin irritations filled with pus. Black heads are clogged pores.