var ctx = document.querySelector("canvas").getContext("2d"),
    grad = ctx.createLinearGradient(0, 0, 0, 150);

grad.addColorStop(0, "red");     // start of red
grad.addColorStop(1/3, "red");   // end of red at 1/3

grad.addColorStop(1/3, "gold");  // start of gold at 1/3
grad.addColorStop(2/3, "gold");  // end of gold at 2/3

grad.addColorStop(2/3, "blue");  // start of blue at 2/3
grad.addColorStop(1, "blue");    // end of blue at 3/3

// Fill a triangle:
ctx.moveTo(75, 0); ctx.lineTo(150, 150); ctx.lineTo(0, 150);
ctx.fillStyle = grad;
ctx.fill();

<canvas/>

var ctx = document.querySelector("canvas").getContext("2d"),
    grad = ctx.createLinearGradient(0, 0, 0, 150),
    step = grad.addColorStop.bind(grad), // function reference to simplify
    dlt = -3, y = 150;

step(0, "red");     // start of red
step(1/3, "red");   // end of red at 1/3
step(1/3, "gold");  // start of gold at 1/3
step(2/3, "gold");  // end of gold at 2/3
step(2/3, "blue");  // start of blue at 2/3
step(1, "blue");    // end of blue at 3/3

// store a triangle path - we'll reuse this for the demo loop
ctx.moveTo(75, 0); ctx.lineTo(150, 150); ctx.lineTo(0, 150);

(function loop() {
  ctx.globalCompositeOperation = "copy";  // will clear canvas with next draw

  // Fill the previously defined triangle path with any color:
  ctx.fillStyle = "#000";  // fill some solid color for performance
  ctx.fill();
  
  // draw a rectangle to clip the top using the following comp mode:
  ctx.globalCompositeOperation = "destination-in";
  ctx.fillRect(0, y, 150, 150 - y);

  // now that we have the shape we want, just replace it with the gradient:
  // to do that we use a new comp. mode
  ctx.globalCompositeOperation = "source-in";
  ctx.fillStyle = grad;
  ctx.fillRect(0, 0, 150, 150);
  
  y += dlt; if (y <= 0 || y >= 150) dlt = -dlt;  
  requestAnimationFrame(loop);
})();

<canvas/>

var ctx = document.querySelector("canvas").getContext("2d"),
    tcanvas = document.createElement("canvas"),    // to cache triangle
    tctx = tcanvas.getContext("2d"),
    grad = tctx.createLinearGradient(0, 0, 0, 150),
    step = grad.addColorStop.bind(grad), // function reference to simplify
    dlt = -3, y = 150;

step(0, "red");     // start of red
step(1/3, "red");   // end of red at 1/3
step(1/3, "gold");  // start of gold at 1/3
step(2/3, "gold");  // end of gold at 2/3
step(2/3, "blue");  // start of blue at 2/3
step(1, "blue");    // end of blue at 3/3

// draw triangle to off-screen canvas once.
tctx.moveTo(75, 0); tctx.lineTo(150, 150); tctx.lineTo(0, 150);
tctx.fillStyle = grad; tctx.fill();

(function loop() {
  ctx.clearRect(0, 0, 150, 150);

  // draw clipped version of the cached triangle image
  if (150-y) ctx.drawImage(tcanvas, 0, y, 150, 150 - y, 0, y, 150, 150 - y);

  y += dlt; if (y <= 0 || y >= 150) dlt = -dlt;  
  requestAnimationFrame(loop);
})();

<canvas/>

var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;

var points=[];
points.push({x:100,y:50});
points.push({x:150,y:150});
points.push({x:50,y:150});
points.push({x:100,y:50});

drawPoints(points);

function drawPoints(pts){
  var minY= 100000;
  var maxY=-100000;
  ctx.save();
  ctx.beginPath();
  for(var i=0;i<pts.length;i++){
    var p=pts[i];
    if(i==0){
      ctx.moveTo(p.x,p.y);
    }else{
      ctx.lineTo(p.x,p.y);
    }
    if(p.y<minY){minY=p.y;}
    if(p.y>maxY){maxY=p.y;}
  }
  ctx.stroke();
  ctx.clip();

  var height=maxY-minY;
  ctx.fillStyle='green';
  ctx.fillRect(0,minY,cw,minY,height/3);
  ctx.fillStyle='blue';
  ctx.fillRect(0,minY+height/3,cw,height/3);
  ctx.fillStyle='red';
  ctx.fillRect(0,minY+height*2/3,cw,height/3);
  ctx.restore();
}

body{ background-color: ivory; }
#canvas{border:1px solid red;}

<canvas id="canvas" width=300 height=300></canvas>