Accepted answer

You can calculate the actual distance of each point to the point of interest and determine points color based on this distance like:

var threshold=...
var p =...'circle').each(function(d){
    var x = p.x - d.x;
    var y = p.y - d.y;
    d.distance = Math.sqrt(x*x + y*y);
}).attr('fill', function(d){
    return d.distance < threshold? 'red' : 'blue'

Ps. Sorry, answered from mobile

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