Yes, it is possible.
Replace the png-file (Version 2.8):
Login-Page: ../PerformanceCenter/sso/webapps/sso/Images/CA_gray_logo_32x32.png
Main-Page: ../PerformanceCenter/PC/webapps/pc/css/CA-Blue/Images/CA_gray_logo_32x32.png
We are using the Themes "gray".
For Version 3.1 Beta I repleaced the following files:
Login-Page: ../PerformanceCenter/sso/webapps/sso/Images/CA_gray_logo_45x37.png
Main-Page: ../PerformanceCenter/PC/webapps/pc/css/CA-Blue/Images/CA_gray_logo_45x37.png
It has been working for several Versions
Why the time is gray and sometimes blue I can not say. Perhaps the concept was not fully implemented.