Particle transparency question

Hi,
Using Fusion 8 Beta 2 on mac so this may be the reason.
New to Fusion. When I try and make particles transparent they become additive. Under 'Merge Controls' there's an 'additive - subtractive' slider that seems to give control over this but it has no effect.
Making smoke and trying to lower the opacity of bitmap particles without them becoming additive. Going for subtle smoke...
Any ideas?
Thanks!
Using Fusion 8 Beta 2 on mac so this may be the reason.
New to Fusion. When I try and make particles transparent they become additive. Under 'Merge Controls' there's an 'additive - subtractive' slider that seems to give control over this but it has no effect.
Making smoke and trying to lower the opacity of bitmap particles without them becoming additive. Going for subtle smoke...
Any ideas?
Thanks!