Adding additional text to the date picker


I am new to frappe and ERPNext.
I would like to add an extra text to the DatePicker next to every day number.
That means, I would like to get date, year, month and based on that add an additional text to the day numbere.e.g.

12 (blue) 13 (green) etc

and I wish this appears in the text box when that date is chosen and be printed when I print that page.

Your help is much appreciated. Thank you in advance.

Kind regards