When selling your device diagnosing your device both before and after the repair or Colour Lab service, your personal data is not accessed by our engineers.
The diagnosis procedure tests the functions and features of the device to make sure they are fully working and this can only be done with an unlocked device.
If any data on your device goes missing during it's time with us we are not liable as per our terms and conditions which are agreed to when placing your order (terms and conditions)
Please see Apple's Support Article which explains how to backup and restore your data.
It is your responsibility to make sure all of your data is backed up and safe.