Browser fingerprinting protection can cause the RGB values of image data returned by context.getImageData() to be randomly off by a little bit (usually -1 or 1). So instead of solid black being rgba(0, 0, 0, 255) as expected, it might be rgba(0, 1, 0, 255) or similar.

The actual colors visible on the canvas are still accurate, which you can confirm by taking a screenshot and sampling the color in an image editor. It is only the pixels in the image data array (Uint8ClampedArray) that are off.

This problem can happen even with a perfectly "clean" canvas that only has one pixel that is definitely black:

let canvas = document.createElement("canvas");
canvas.width = 1;
canvas.height = 1;
let context = canvas.getContext("2d");
context.fillStyle = "rgba(0, 0, 0, 255)";
context.fillRect(0, 0, 1, 1);
let data = context.getImageData(0, 0, 1, 1).data;

console.log(data);
// Expected result: [ 0, 0, 0, 255 ]
// Fingerprinting protection can cause something like: [ 0, 2, 1, 255 ]

It's Not Color Space

If you search for why this is happening, you'll probably run across a lot of results about color space. You can indeed set the colorSpace property of the image data, but it probably won't help.

Strict Settings

This issue with fingerprinting protection usually happens if you have "strict" privacy settings and are running off a local virtual host. Firefox and other browsers with strict settings may treat the virtual host as a "suspected fingerprinter", since they probably don't recognize the domain, which enables the fingerprinter protection and adds the noise to your image data.

Solution

To fix this behavior on your local machine, you may need to disable "suspected fingerprinters", or set it to "only in private windows". Note that this only affects your computer. It is possible users browsing your website or using your web app could run into this behavior, causing your JavaScript to behave differently than expected. Luckily, this seems to only happen if your domain is viewed as suspicious, or is a known fingerprinting domain.