Reduce transparency and Lock-screen Background I'm running El captain public beta but I think this also applies to macs running Yosemite. I really like the look of the reduce transparency + increase contrast in Yosemite/El Captain. Unfortunately those options also force the lock screen background to be a boring grey colour instead of the blurred version of my desktop background :(

I am aware that you can change the lock/login screen background by navigating to /Library/Caches and changing a file. I checked and the file is as it should be - the blurred version of my desktop background.

Is there a way to set the lock screen background to something other than the grey colour while still keeping reduce transparency and increase contrast on? Perhaps something to do with changing system files?

EDIT: I did some research and it seams like this behaviour is unique to El Captain. Yosemite has the desired behaviour of the lock screen being the blurred background even with reduce transparency and increased contrast turned on
I have had this question as well for a long time and my answer is — as far as I can tell and have tried and researched in other posts and fora — there is no way to do so. Hopefully that assertion will motivate someone to try and find an answer that proves me wrong. 

As of 2017 the issue you are having is also present in MacOS Sierra — to respond to the claim that it is unique to El Capitan — making the issue still relevant. I have asked this question myself recently on apple.stackexchange but nobody has responded as of yet.