Black…

[Elvis Deane]

If the Transparency gradient is set to Link Transparency to Color, and you choose black in your Color gradient, your particles will become transparent. Select “Use full gradient” under Transparency, and then change the colour to black. After that, you can adjust the Transparency gradient independently.

—
Elvis Deane!

The Apprentice Magician’s Guide to particleIllusion

Wilbur of Wumbaberry

[Alan Lorence]

Also, if your particles are “intense”, then the color also contributes to transparency — “intense” particles that are black won’t be visible no matter what the transparency gradient does (except maybe if “preserve color” is used — can’t remember).

Alan.

http://www.wondertouch.com