Can someone please help me, I need to achieve the color pistachio for my buttercream.

Thank you in advance!!!

Use gel food (green) color a little at a time to you get your color your looking for

Pistachio has a touch of yellow and a tiny touch of blue added to grass green.

Pistachio is a very light green so do go VERY! light on adding color.  You can always add more if needed but if it goes too dark/aka:GREEN! then it would take a ton of more white icing to tone it down :)