GetColorByDay()

index.js

const oneDay=1000*60*60*24;
const firstMondayDay = new Date("June 3, 2019");
const colors = ["Red", "Orange", "Yellow", "Green", "Blue", "Purple", "Black"];

function checkFirstMonday(day) {
    if (day.getDay() != 1) {
        throw "The first Monday is not set correctly.";
    }
}

function getColorByDay(day) {

    if (day.getDay() == 0 || day.getDay() == 6) {
        throw "It is weekend. No cars are made.";
    }
    if (day.getTime() < firstMondayDay.getTime()) {
        throw "Please input a day after " + firstMondayDay;
    }
    if (day.getTime() > (firstMondayDay.getTime()+30*oneDay)) {
        throw "Please input a day less than 30 days from " + firstMondayDay;
    }
    let deltaTime = day.getTime() - firstMondayDay.getTime();
    let deltaDays = Math.round(deltaTime / oneDay);
    let weekNo = Math.round(deltaDays / 7);

    return colors[(deltaDays - weekNo*2) % colors.length];
}




// TEST
try {
    checkFirstMonday(firstMondayDay);

    let day = new Date("June 3, 2019");
    let color = getColorByDay(day);
    console.log(color);

    day = new Date("June 4, 2019");
    color = getColorByDay(day);
    console.log(color);
    
    day = new Date("June 10, 2019");
    color = getColorByDay(day);
    console.log(color);
    
} catch (e) {
    console.log(e);
}