If I am reading your post correctly, place your mouse in the bottom right of the screen--then click settings--at the bottom click change pc settings--personalize--and you can change lock screen, start screen and account picture there..Hope that helps
afaik, the login screen is the same colour as the accent colour you choose for the Metro background
Oh I did not know it uses the same color as the start screen background. I usually never see it, but I did yesterday for some reason I guess because it installed windows update. Is it still purple then, or was I seeing things?
The purple is the "default" color, so even once you change your color, you may still see the purple now and then if different people use the computer. But overall, once you set your accent color for your profile, that's the only color you should see mainly.