How can I create a customised time slot calendar?

I'd actually need a negative example to be completely sure but as I see it there must be either a CSS class to make the cells blue or to make them white. Either way, you can actually do something along the lines of

public static WebElement getFreeSlot(String colorCode)
List slots = driver.findElement(By.cssSelector(".day-body .hour-wrapper"));
for(WebElement slot : slots){
    String colour = slot.getCssValue("background-colour");
    if(color.equals(colorCode))
        return slot;
}
return null;
}

Just pass it the colour code for your background (should be just white but look it up to be safe) and you should get the first, white time slot calendar.