Proving that the clique cover problem is in NPC by reducing from k-coloring
In the interests of putting up an explicit answer, your approach is correct. The proof is sketched on the Wikipedia page for Clique Cover.

The reduction itself is fairly straightforward. The reasoning is that if a graph is $k$-colourable, then it can be partitioned into $k$ independent sets (one for each colour class), then we just exploit the normal reduction between Independent Set and Clique by taking the complement graph (we swap edges for non-edges and vice versa), so any independent set becomes a clique. So if $G$ is $k$-colourable, it can be partitioned into $k$ independent sets and hence $\overline{G}$ can be partitioned into (i.e. covered by) $k$ cliques (but not necessarily by $k$-cliques ;) ). Conversely if $\overline{G}$ can be covered by $k$ cliques, $G$ has a partition into $k$ independent sets, and hence is $k$-colourable.