Gray screen after installing Yosemite
If you go into the screen that lets you re-download the OS and also gives you the ability to see disk utility command + r during bootup.

Run disk utility and select the Mac Harddriive [not the OS partition] and run the Repair Disk Permissions feature and then restart the computer and you should have a permanent solution to the black/gray screen.

RECAP [make sure computer is off]:


Turn on Mac
Hold command + R as system boots up
When in recovery mode select a Computer Harddriive [not the OS partition]
Run Repair Disk Permissions
Reboot