If you mean make the color more or less transparent/opaque, and since Footprint is a continuous (not discrete) value, you can pass it to the alpha aesthetic like this:
ggplot(data=w, aes(x=Continent, alpha=Footprint))
You could still use continent as fill as you have it already.
ggplot(data=w, aes(x=Continent, fill = Continent, alpha=Footprint))