I have recently planted two weeping cherries. I am noticing the leaves are not as dark green as my neighbors

I think you might need to do a soil test. There may be some nutrients lacking. I wouldn't fertilize without first doing that.

Newt

First, the tree was recently transplanted. Was your neighbors also a recent addition? It may not have established yet.

Second, also test the soil pH and make sure it is appropriate for the plant. High pH can cause an iron or manganese deficiency which makes leaves less green. I think in the case of cherry it's iron.