NetLogo 7.0.0-beta2:

palette:set-transparency

palette:set-transparency number

Set the color of the agent to the RGBA color with alpha value equivalent to the transparency number and the RGB content of the color of the turtle or link. An error message results if number is not in the range 0 to 100.

Example:

ask turtles [palette:set-transparency 30]

Take me to the full Palette Extension Dictionary