  A fall $k$-coloring of a graph $G$ is a proper $k$-coloring of $G$ such that
each vertex of $G$ sees all $k$ colors on its closed neighborhood. We denote
${\rm Fall}(G)$ the set of all positive integers $k$ for which $G$ has a fall
$k$-coloring. In this paper, we study fall colorings of lexicographic product
of graphs and categorical product of graphs and answer a question of \cite{dun}
about fall colorings of categorical product of complete graphs. Then, we study
fall colorings of union of graphs. Then, we prove that fall $k$-colorings of a
graph can be reduced into proper $k$-colorings of graphs in a specified set.
Then, we characterize fall colorings of Mycielskian of graphs. Finally, we
prove that for each bipartite graph $G$, ${\rm Fall}(G^{c})\subseteq \{\chi
(G^{c}) \}$ and it is polynomial time to decision whether or not ${\rm
Fall}(G^{c})=\{\chi (G^{c}) \}$.
