It is quite simple using the
noise function, one the simplest things you can do with it is
color = noise(x, y, time) for each pixel on the canvas, usually you make it grayscale but this time I'm generating 3 of them for each color component (red, green, blue) with a different seed. It looks pretty cool, if you ask me.
On that, I recommend p5.js, or its big sister Processing, if you want to learn code and do some cool artsy stuff with it, I've been recommending to a bunch of people lately :) If you want to learn and you're a complete beginner, there's a series about that on The Coding Train on YouTube.
Edit (26/11): Updated the post with code and some explanations.