  For studying topological obstructions to graph colorings, Hom-complexes were
introduced by Lov\'{a}sz. A graph $T$ is called a test graph if for every graph
$H$, the $k$-connectedness of $|Hom(T, H)|$ implies $\chi (H)\geq k + 1 +
\chi(T)$. The proof of the famous Kneser conjecture is based on the fact that
$\mathcal{K}_2$, the complete graph on $2$ vertices, is a test graph. This
result was extended to all complete graphs by Babson and Kozlov. Their proof is
based on generalized nerve lemma and discrete Morse theory.
  In this paper, we propose a new topological lower bound for the chromatic
number of a special family of graphs. As an application of this bound, we give
a new proof of the well-known fact that complete graphs and even cycles are
test graphs.
