The sherry casks give off the darker color. Port casks make it even darker but I don't like whisk(e)y from port barrels myself.
Johnny Walker Black Label is a whisky with a light natural color, being blended from Caol Ila, Talisker and grain whisky. The dark color comes from liberal quantities of E150a (caramel). Caramel dye looks quite different from the reddish brown tint of PX or Port casks or the yellow of the oak. So Black Label is a pretty good sample of a color a whisky should not have. Some very, very old whiskies have similar colors to Black Label, but they are exceptions - no bottler in their right mind would add E150 to a Brora 50yo.
Never judge a whisky by it's (lack of) color.