在 HTML 中单击按钮时 JS 变量不更新

我正在网页上制作绘图/画布应用程序。我想通过按钮更改颜色，但变量没有更新颜色。我调试了代码并注意到它确实更新了，但在画布上绘制时颜色本身没有改变。

.HTML：

<div id="sketch">
  <canvas id="paint"></canvas>
</div>
<button onClick="changecolour('blue')">Blue</button>
<button onClick="test()">DEBUG</button> 

JavaScript：

var canvas = document.querySelector('#paint');
var ctx = canvas.getContext('2d');
var sketch = document.querySelector('#sketch');
var sketch_style = getComputedStyle(sketch);
canvas.width = parseInt(sketch_style.getPropertyValue('width'));
canvas.height = parseInt(sketch_style.getPropertyValue('height'));
var mouse = {x: 0, y: 0};
canvas.addEventListener('mousemove', function(e) {
  mouse.x = e.pageX - this.offsetLeft;
  mouse.y = e.pageY - this.offsetTop;
}, false);
var colour = "black";
function changecolour(choice){
    colour = choice;    
}
function test(click){
    alert("You choose " + colour);
}
ctx.lineWidth = 5;
ctx.lineJoin = 'round';
ctx.lineCap = 'round';
ctx.strokeStyle = colour;
canvas.addEventListener('mousedown', function(e) {
    ctx.beginPath();
    ctx.moveTo(mouse.x, mouse.y);
    canvas.addEventListener('mousemove', onPaint, false);
}, false);
canvas.addEventListener('mouseup', function() {
    canvas.removeEventListener('mousemove', onPaint, false);
}, false);
var onPaint = function() {
    ctx.lineTo(mouse.x, mouse.y);
    ctx.stroke();
};