So I’m kinda new and I’ve wanted to add a sort of Ambiant Dust in this scene :
As you see they don’t show up… I’ve seen ( How to show dust particles - FX - Epic Developer Community Forums ) that I should plug the Particle Color into Emissive and etting the material to unlit to do the job but it still doesn’t show up :
Thanks for your help 
Try increasing the opacity and size of the particles.
That and you realize your only plugging the red channel into the opacity multiplier. Its multiplying the opacity of the particle color by the red channel of the UV texture. You either want the top or bottom pin unless you have a specific reason for that.
Loook at the dust particle system from the starter content for reference. They are HARD to see.
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I have no idea about what you are saying ^^'…I had managed to make dust appear by increasing the opacity
In this picture the red pin is connected to the multiply node Imgur: The magic of the Internet
So your multiplying the ParticleColors opacity * the amount of data in the red color channel of the dust texture then putting that value in opacity. You should have either all color (red,green, blue) by using the top pin of the texture instead, or the opacity by using the bottom pin.
Using the red channel for opacity is fine, you need a constant 1 for opacity anyway and red channel gives you that.
Thank’s it’s actually working !