Каждый сегмент должен быть окрашен.
function qsa(sel,par=document){return par.querySelectorAll(sel)}
window.addEventListener('load', onLoaded, false);
function onLoaded(evt)
{
draw();
}
class vec2d
{
constructor(x=0,y=0)
{
this.x = x;
this.y = y;
}
}
function draw()
{
var verts = [ new vec2d(20,140), new vec2d(120, 10), new vec2d(220,140) ];
var colours = ['red', 'green', 'blue'];
let can = qsa('canvas')[0];
let ctx = can.getContext('2d');
var numLineSegs = verts.length;
for (var lineSegIndex=0; lineSegIndex<numLineSegs; lineSegIndex++)
{
var index1 = lineSegIndex;
var index2 = (lineSegIndex+1)%verts.length;
ctx.beginPath();
ctx.strokeStyle = colours[index1];
ctx.moveTo(verts[index1].x, verts[index1].y);
ctx.lineTo(verts[index2].x, verts[index2].y);
ctx.stroke();
}
ctx.closePath();
}
<canvas width=512 height=512></canvas>