// linear gradient from start to end of line
var grad= ctx.createLinearGradient(50,50,150,150);
grad.addColorStop(0,"red");
grad.addColorStop(1,"green");

ctx.strokeStyle = grad;

ctx.beginPath();
ctx.moveTo(50,50);
ctx.lineTo(150,150);

ctx.stroke();

在这里看到它：

http://jsfiddle.net/9bMPD/