SOURCE

let r=0;
let g=0;
let b=0;
function setup(){
    createCanvas(500,150);
}

function draw(){
    if(keyIsPressed){
        if(key=="r"){
            r=0;
            g=255;
            b=0;
        }
        else if(key=="g"){
            r=0;
            g=0;
            b=255;
        }
        else if(key=="b"){
            r=255;
            g=0;
            b=0;
        }
    }

    fill(r,g,b);

}

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