  \qquad A \emph{coloring} of a digraph $D=(V,E)$ is a coloring of its vertices
following the rule: Let $uv$ be an arc in $D$. If the tail $u$ is colored
first, then the head $v$ should receive a color different from that of $u$. The
\emph{dichromatic number} $\chi_d(D)$ of $D$ is the minimum number of colors
needed in a coloring of $D$. Besides obtaining many results and bounds for
$\chi_d(D)$ analogous to that of chromatic number of a graph, we prove
$\chi_d(D)=1$ if $D$ is acyclic. New notions of sequential colorings of
graphs/digraphs are introduced. A characterization of acyclic digraph is
obtained interms of $L$-matrix of a vertex labeled digraph.
