Again this is fairly self-explanatory. Below are the result and the corresponding code. If it should not work on mobile, just click this link. It will open up in a new browser.
Color of rectangle changes when you move the mouse
var x;
function setup() {
createCanvas(400, 400);
}
function draw() {
x = map(mouseX, 0, 400, 0, 255)
y = map(mouseY, 0, 400, 255, 0)
background(225);
//rectangle
fill(x)
noStroke()
rectMode(CENTER)
rect(200, 200, 50, 50)
//console.log(x)
}