The set up here is pretty straight forward. Take a shape, get its volume, generate leaves within that volume and set the vertex color based on how near the surface the leaf is.
My solution was to use geometry proximity node to measure the instance's distance from the original volume's surface. When I plug the output of the geometry proximity into a map range node and then into a Scale instances node, I can see its working correctly with surface leaves being smaller. However, when I plug that same output into a color ramp I do not get the expected output. Instead each leaf seems to have the same vertex color regardless of where they are in the volume.