Did you follow the instructions in the link? Here they are so that way you don't have to search again:
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sudo xhost +SI:localuser:lightdm
sudo su lightdm -s /bin/bash
gsettings set com.canonical.unity-greeter draw-grid false
exit
The first line is extremely important. So is the second line. Those prepare the entire process. The third line can be changed to whatever you want. I set my background to black by doing:
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gsettings set com.canonical.unity-greeter background-color '#000000'
It isn't purple for me. If ubuntu tweak now supports linux mint and changes all the stuff with a GUI, that's the easiest way to do it. In fact I will try that now and see if it works for me.
EDIT: It does almost everything but change the background color. So the only way I know of to fix it is the above steps. I do have a SSD, so there is only about a half second of the color before the background comes up. That brings up another point. Did you change the background from the purple ubuntu one? I remember that was an issue the first time I installed it.