Property:
addColorStop:

Canvas:
your browser does not support the canvas tag
Code:
var c = document.getElementById('myCanvas');
var ctx = c.getContext('2d');
var grd = ctx.createLinearGradient(0,0,150,0);
grd.addColorStop(0.3,'red');
grd.addColorStop(0.7,'green');
ctx.fillStyle = grd;
ctx.fillRect(0,0,150,100);
Click the property values above to see the result
W3Schools.com - Play it