SOURCE

let h=150;
let point=15;
function setup() {
    createCanvas(600,600);
    colorMode(HSB,360,50,50);
}

function draw() {
    if(keyIsPressed){
        if(key=='r'){
            h-=5;
        }
        else if(key=='g'){
            h+=5;
        }
        else if(key=='b'){
            h=150;
        }

    }
    if(mouseIsPressed){
        strokeWeight(point);
        stroke(h,90,90);
        line(mouseX,mouseY,pmouseX,pmouseY);
    }
}

<script src='https://cdn.jsdelivr.net/npm/p5@1.1.9/lib/p5.min.js'></script>