Stuff
public text v1 · immutable1) Go to Utilities > Disk Utility
2) Select your partition that you want to be OSX and go to the Erase tab
3) For Volume Format, select Mac OS Extended (Journaled)
4) Click Erase. Now the partition should not be grey, it should be black to indicate that it is active.
5) Close out of the Disk Utility and move onwards with installation.