Get a pixel from HTML Canvas?

Yes sure, provided you have its context. (See how to get canvas context here.)

var imgData = context.getImageData(0,0,canvas.width,canvas.height)
// { data: [r,g,b,a,r,g,b,a,r,g,..], ... }

function getPixel(imgData, index) {
  var i = index*4, d = imgData.data
  return [d[i],d[i+1],d[i+2],d[i+3]] // Returns array [R,G,B,A]
}

// AND/OR

function getPixelXY(imgData, x, y) {
  return getPixel(imgData, y*imgData.width+x)
}

---

PS: If you plan to mutate the data and draw them back on the canvas, you can use subarray

var
  idt = imgData, // See previous code snippet
  a = getPixel(idt, 188411), // Array(4) [0, 251, 0, 255]
  b = idt.data.subarray(188411*4, 188411*4 + 4) // Uint8ClampedArray(4) [0, 251, 0, 255]

a[0] = 255 // Does nothing
getPixel(idt, 188411) // Array(4) [0, 251, 0, 255]

b[0] = 255 // Mutates the original imgData.data
getPixel(idt, 188411) // Array(4) [255, 251, 0, 255]

// Or use it in the function
function getPixel(imgData, index) {
  var i = index*4, d = imgData.data
  return imgData.data.subarray(i, i+4) // Returns subarray [R,G,B,A]
}

You can experiment with this on http://qry.me/xyscope/, the code for this is in the source, just copy/paste it in the console.

function GetPixel(context, x, y)
{
    var p = context.getImageData(x, y, 1, 1).data; 
    var hex = "#" + ("000000" + rgbToHex(p[0], p[1], p[2])).slice(-6);  
    return hex;
}

function rgbToHex(r, g, b) {
    if (r > 255 || g > 255 || b > 255)
        throw "Invalid color component";
    return ((r << 16) | (g << 8) | b).toString(16);
}

There's a section about pixel manipulation in the W3C documentation.

Here's an example on how to invert an image:

var context = document.getElementById('myCanvas').getContext('2d');

// Get the CanvasPixelArray from the given coordinates and dimensions.
var imgd = context.getImageData(x, y, width, height);
var pix = imgd.data;

// Loop over each pixel and invert the color.
for (var i = 0, n = pix.length; i < n; i += 4) {
    pix[i  ] = 255 - pix[i  ]; // red
    pix[i+1] = 255 - pix[i+1]; // green
    pix[i+2] = 255 - pix[i+2]; // blue
    // i+3 is alpha (the fourth element)
}

// Draw the ImageData at the given (x,y) coordinates.
context.putImageData(imgd, x, y);

Try the getImageData method:

var data = context.getImageData(x, y, 1, 1).data;
var rgb = [ data[0], data[1], data[2] ];