That's a great looking filly, and yes, she'll darken up. And lighten up and darken up...LOL! My Goldie is all gold and dappled in summer and gets a much darker buckskin coat in winter. It's very fun to watch the changes.
All bay based horses (buckskins are bay + cream) that aren't grey are born with light legs that later darken.
Most foals are born with what's called "foal flaxen" which will shed out. Many buckskins do keep some frosting, but it probably won't be as pronounced as it is now.