  A tree in an edge colored graph is said to be a rainbow tree if no two edges
on the tree share the same color. Given two positive integers $k$, $\ell$ with
$k\geq 3$, the \emph{$(k,\ell)$-rainbow index} $rx_{k,\ell}(G)$ of $G$ is the
minimum number of colors needed in an edge-coloring of $G$ such that for any
set $S$ of $k$ vertices of $G$, there exist $\ell$ internally disjoint rainbow
trees connecting $S$. This concept was introduced by Chartrand et. al., and
there have been very few related results about it. In this paper, We establish
a sharp threshold function for $rx_{k,\ell}(G_{n,p})\leq k$ and
$rx_{k,\ell}(G_{n,M})\leq k,$ respectively, where $G_{n,p}$ and $G_{n,M}$ are
the usually defined random graphs.
