Any time I create a 3d doc of a material with some color assigned, the associated hatch has as pen with number -2, which icon looks like a pie chart and is said to be "RGB". Is there any shorter way to pick -2 pen rather than creating a material with some specific color, and copying the linked 3d hatch properties?
Unfortunately NO. This icon means the colour has been generated from the 3D window and is effected by sun / shading etc., and is not one of the standard 256 pen colours. You can use the syringe (Option-click on Mac, CTRL-click on PC) to pick up an existing shade though.