Manual says about YELLOW TAB:
Tab Change Indicators
The colored tab corner gives information on whether a file is saved or not, and whether the file
has been rendered in Cubase.
White - The file is not modified.
Green - (Audio Editor only) The file uses a decoded file format and is saved.
Red - The file has been modified and changes have not been saved yet.
Yellow - The file has been rendered in Cubase.
Unsaved changes indicator - When you have made changes to a file, an asterisk is displayed next to the file name until you save the file.