After upgrading my OS X 10.10 DP1 VMs to DP2, on two different computers, I found that the menu bar had turned black: well, this is easily corrected, inside the guest, by selecting Reduce transparency in System Preferences ---> Accessibility.
Not sure why that setting works to restore the normal menu bar color, but anyway it works...
Another solution is to turn on Dark Mode:
sudo defaults write /Library/Preferences/.GlobalPreferences AppleInterfaceTheme Dark.
To return to the default light gray graphics, change "Dark" to "Light" and perform a login cycle.