Enumerating topological sorts of a vertex-labeled DAG
One of the simplest way to compute topological ordering is performing a Depth First Search on the given DAG. Different topological orderings can be generated by exploiting the flexibility of choosing the next vertex $v$ to be traversed from the unvisited neighbours of the current vertex $u$.
Since, it is a recursive procedure enumerating all possible traversals (and hence topological orders) would be simple, by choosing different orders in which unvisited neighbours of $u$ are traversed.

Now, in order to limit repeating the same traversal because of similar labels, one can compare the unvisited neighbours $v_1,v_2,...,v_k$ of $u$ having similar labels. Consider two vertices $v_i$ and $v_j$ that have the same unvisited neighbours when the traversal reaches $u$. Surely choosing either of them first would generate the same DFS tree and hence any one of them can be avoided.

Now, comparing the neighbours of all $v_1,...,v_k$ would lead to an overhead of $O(n^2)$ on the total time, but can be performed more efficiently in $\tilde{O}(n)$ by using appropriate data structures.