You want to get the colour of an image and then fill with that colour so the text will be the same colour.

Here's a link to the documentation for the get() method which is what you're looking for.

In order to do this, we use the get(x, y) method to get the pixel at a specific location, this returns us an RGBA value which we can use to fill.

I've put together a really simple example to demonstrate this:

let img;
function preload() {
  img = loadImage('img.jpg');
}

function setup() {
  createCanvas(400, 400);
  image(img, 0, 0);
  let c = get(0, 0);
  fill(c);
}

function draw() {
  background(0);
  image(img, 0, 0);
  textSize(20);
  text('I am the same colour', 100, 350);
}

I'm just grabbing the very first pixel and filling with that. Here's a link to a p5.js sketch so you can see it running.