Black sprites issue

Hi everyone ,
I am having this strange issue with the particle rendered. I am using a sprite sheet 5x5. It has white tweets on a grid. I emitt from a black and white kinect depth camera image. Some of the tweets are rendered darker or even almost as if they have a black multiply layer on them. I tried using the Colour selection mode to single color but it seems to have no difference. I am emitting from luminance.
No lighting in the scene. Any ideas on how to fix that.
Thank you !

There’s a number of ways this could be caused, so I’m not really sure anyone can can offer much useful advice without a project file to look at. if you’ve got a simple scene that shows the issue in isolation, I’d be happy to take a look.

– Ryan