SOURCE

let b=150;
let n=20;

function setup(){
    createCanvas(500,500);
    colorMode(HSB,300,250,90);
}
function draw(){
    if(keyIsPressed){
        if(key=='q'){
            b-=1;
    }
        else if(key=='w'){
            b+=1;
    }
        else if(key=='a'){
            n-=0.5;
    }
        else if(key=='s'){
            n+=0.5;
    }
}
if(mouseIsPressed){
stroke(b,100,100);
strokeWeight(n);
line(mouseX,mouseY,pmouseX,pmouseY);
    }
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js"></script>