Hello and welcome,
Two are possible reasons that your sprite looks more bright in editor.One has to do with Mobile HDR ON.You can find in settings menu.A second reason would be if you have TemporalAAA enabled inside your post volume process.2D image Color problem - Rendering - Epic Developer Community Forums dont know what happened to original OP and if he fixed his problem but in my case it made everything look correct.
Switch wrap to clamp.It helped me in more than a few situations to get rid of some artifacts.
Another thing to consider is : PaperSprite texture settings question - Rendering - Epic Developer Community Forums
It seems that things are not very well optimized for sprites since you either get blockiness or artifacts and washed out image quality.Maybe there is a workaround for but i never received an answer so i do not know.
I hope that something of above will fix your problems.If not post back and maybe will help you out since he is expert. 