  We introduce an up-down coloring of a virtual-link diagram. The
colorabilities give a lower bound of the minimum number of Reidemeister moves
of type II which are needed between two 2-component virtual-link diagrams. By
using the notion of a quandle cocycle invariant, we determine the necessity of
Reidemeister moves of type II for a pair of diagrams of the trivial
virtual-knot. This implies that for any virtual-knot diagram $D$, there exists
a diagram $D'$ representing the same virtual-knot such that any sequence of
generalized Reidemeister moves between them includes at least one Reidemeister
move of type II.
