Colored range queries

Let $S$ be a set of $n$ points and assume that each point has a color, where different points may have the same color. We want to store the points of $S$ in a data structure such that for any axes-parallel query box $B$, we can report the distinct colors of all points that are in $B$. The query time should be sensitive to the number of colors; thus, if many points of the same color $c$ are contained in $B$, then this color $c$ is “counted” only once. I will present some old results for this problem. I will also present some variations which are still open.