function contains(arr, el) {
    for(let i = 0; i < arr.length; i++) {
        if(arr[i] === el) {
            return true;
        }
    }
    return false;
}

function getThirdColour2(vitamin, newColour) {
    arr = [0,1,2];
    arr = arr.filter(item => !contains([vitamin, newColour], item));
    return arr[0];
}