Edit in JSFiddle

function lerp(a, b, p) {
    return (b - a) * p + a;
}

var canvas = document.getElementById("scratch");

// canvas dimensions
var width = parseInt(canvas.style.width);
var height = parseInt(canvas.style.height);

// retina
var dpr = window.devicePixelRatio || 1;
canvas.width = width * dpr;
canvas.height = height * dpr;
canvas.getContext("2d").scale(dpr, dpr);

// simulation
var sim = new VerletJS(width, height, canvas);
sim.friction = 1;
sim.highlightColor = "#fff";
//sim.gravity = new Vec2(0, 0);

// entities
var min = Math.min(width, height) * 0.5;
var segments = 20;
var cloth = sim.cloth(new Vec2(width / 2, height / 3), min, min, segments, 6, 0.9);

cloth.drawConstraints = function (ctx, composite) {
    var stride = min / segments;
    var x, y;
    for (y = 1; y < segments; ++y) {
        for (x = 1; x < segments; ++x) {
            ctx.beginPath();

            var i1 = (y - 1) * segments + x - 1;
            var i2 = (y) * segments + x;

            ctx.moveTo(cloth.particles[i1].pos.x, cloth.particles[i1].pos.y);
            ctx.lineTo(cloth.particles[i1 + 1].pos.x, cloth.particles[i1 + 1].pos.y);

            ctx.lineTo(cloth.particles[i2].pos.x, cloth.particles[i2].pos.y);
            ctx.lineTo(cloth.particles[i2 - 1].pos.x, cloth.particles[i2 - 1].pos.y);

            var off = cloth.particles[i2].pos.x - cloth.particles[i1].pos.x;
            off += cloth.particles[i2].pos.y - cloth.particles[i1].pos.y;
            off *= 0.25;

            var coef = Math.round((Math.abs(off) / stride) * 255);
            if (coef > 255) coef = 255;

            ctx.fillStyle = "rgba(" + coef + ",0," + (255 - coef) + "," + lerp(0.25, 1, coef / 255.0) + ")";

            ctx.fill();
            ctx.closePath();
        }
    }

    var c;
    for (c in composite.constraints) {
        if (composite.constraints[c] instanceof PinConstraint) {
            var point = composite.constraints[c];
            ctx.beginPath();
            ctx.arc(point.pos.x, point.pos.y, 1.2, 0, 2 * Math.PI);
            ctx.fillStyle = "rgba(255,255,255,1)";
            ctx.fill();
            ctx.closePath();
        }
    }
}

cloth.drawParticles = function (ctx, composite) {
    // do nothing for particles
}

// animation loop
var legIndex = 0;
var loop = function () {
    sim.frame(16);
    sim.draw();
    requestAnimFrame(loop);
};

loop();

<canvas id="scratch" style="width: 800px; height: 500px; background: #000;"></canvas>