To me, those leaves look chlorotic (pale green/ yellow, with dark veins).
"Chlorosis is a yellowing of leaf tissue due to a lack of chlorophyll. Possible causes of chlorosis include poor drainage, damaged roots, compacted roots, high alkalinity, and nutrient deficiencies in the plant. Nutrient deficiencies may occur because there is an insufficient amount in the soil or because the nutrients are unavailable due to a high pH (alkaline soil). Or the nutrients may not be absorbed due to injured roots or poor root growth.
The lack of iron is one of the more common nutrients associated with chlorosis. Manganese or zinc deficiencies in the plant will also cause chlorosis" https://urbanext.illinois.edu/focus/inde ... =chlorosis
Are you fertilizing your sapling? Does the fertilizer you use have iron and other minerals in it?
And check what applestar said about drainage. Poor drainage can cause this or compacted roots. In the spring you may need to re-pot it into a larger pot. Your container does look small for the size of the tree.