I wish saw this a few months ago
It happens if you are using some screen adjusting app like Screen Adjuster. You will have to turn it off to make the install option activate.
Yup, I have a Note 2 - the OLED screen can be problematic for my artwork without toning down the red. I imagine I'm not the only artist to use a Note as a quick and dirty, extemporaneous and backup portfolio.
Sure enough "Screen Adjuster" which allows RGB color balancing also makes the Install button unclickable when directly installing APKs. Drove me nuts - always found a temporary solution - rebooting, stopping Play Services, clearing caches for Services and the Play app (plus Aptiode, etc.) but these were hit and miss. Turned it off and it worked without any other hoops to jump through.
Glad I came across this - I don't usually install APKs at this point as my Note 2's apps/uses are pretty stable at this point, but for those days when you have 15 Xposed Framework APKs to install... and. clicking! stabbing! the damn! install button. does. nothing. ...yeeesh.
Figured this out from an Phandroid thread that listed similar apps:
Easyeyez Screen Filter
Twilight Filter
...and other screen dimming filter apps. The post is long because it's been a hassle for a long time... and hopefully to make it a bit easier for someone to find, including the above apps.
God that was annoying. Now to fix my memleak/lag issues.