The colors of the circles should match the color of the background cells.
<!DOCTYPE html>
<meta charset="utf-8">
<style>
circle,
path {
stroke: #000;
}
path {
fill-opacity: .1;
}
path:hover {
fill-opacity: .2;
}
</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>
var points = [
[200, 200],
[760, 300]
];
var color = d3.scale.category10();
var svg = d3.select("body").append("svg")
.attr("width", 960)
.attr("height", 500);
svg.selectAll("path")
.data(d3.geom.voronoi(points))
.enter().append("path")
.style("fill", function(d, i) { return color(i); })
.attr("d", function(d) { return "M" + d.join("L") + "Z"; });
svg.selectAll("circle")
.data(points)
.enter().append("circle")
.style("fill", function(d, i) { return color(i); })
.attr("transform", function(d) { return "translate(" + d + ")"; })
.attr("r", 4.5);
</script>