SOURCE

let h=200;
let point=10;
function setup() {
    createCanvas(500,500);
    colorMode(HSB,360,100,100);
}

function draw() {
    if(keyIsPressed){
        if((key=='r')||(key=='R')){
            h-=1;
        }
        else if((key=='g')||(key=='G')){
            h+=1;
        }
                else if((key=='b')||(key=='B')){
            h+=1;
        }
         

    }
    if(mouseIsPressed){
        strokeWeight(point);
        stroke(h,100,100);
        line(mouseX,mouseY,pmouseX,pmouseY);
    }
}

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