You do not need to collect your bags again.
As you are flying on one ticket; the agent will check your bags all the way to your final destination - you will not need to collect your bags and check-in again in Istanbul.
You will also be given boarding passes for both legs of your journey at your check-in in Colombo.
If you purchased two separate tickets (even if they were both on Turkish Airlines):
Colombo - Istanbul
Istanbul - Berlin
Then you will have to ensure:
- You have a valid visa for Turkey
- You will have to enter into Turkey, collect your luggage.
- You will have to check-in again in Turkey for your second leg.
Unless you booked the tickets manually - the above does not apply to you. If you booked your flight through a travel agent or through the turkish airlines website, you are on one ticket where your bags will be checked in to your final destination.
For more information, you can browse the excellent transit information page on the Turkish Airlines website.