umap highlighting two different models
By default, DimPlot colors the cells by their "identity class", stored in Ident(scData) in your case. This indeed contains the clustering information.

However, you can you use the group.by argument for DimPlot to pass another column from scData@meta.data to color cells by. In your case you would need a metadata column that stores the ptx/human information.

The merge function automatically stores the original "project" information of the individual Seurat objects that you combine under orig.ident in the metadata. Since you set the project names to either "ptx" or "human", the orig.ident column should contain the information you need.

Something like this should work:

DimPlot(scData, reduction = "umap", group.by = "orig.ident", label = TRUE)