Starkly describes something that's absolutely, undeniably clear. Because Harry Potter's skin was so pale, his lightning bolt scar stood out starkly against his forehead.
The adverb starkly is good for talking about things that are visually severe or harsh, like the silhouette of a crow that stands starkly against the gray sky. You can also use it for a more figurative kind of harshness: "The Australian wildfires starkly illustrate the effects of climate change." We can trace this word back to a root (which it shares with stern) that means "stiff."