JavaScript

• jQuery

• You don't need JavaScript for that
• Everyone has JavaScript, right?

var color_years = [];
color_years[year] = "red";
 
if (!(year in color_years)) {
    color_years[year] = "blue";
}

Source

var o = JSON.parse('["2020-05-05", "2020-05-06", "2020-05-07", "2020-05-08", "2020-05-09"]')