NetLogo 7.0.0-beta2:

palette:with-alpha

color palette:with-alpha number

Reports an RGBA color with alpha component equal to number and the RGB content of color. An error message results if alpha is not in the range from 0 to 255.

Example:

show [12 34 55 6] palette:with-alpha 99 ;; [12 34 55 99]

Take me to the full Palette Extension Dictionary