Change group alpha but not color?

[Kaminski]

Hi all,
I am having trouble changing the alpha values of all nodes in a group without changing the color. Any advice?

import viz
import vizshape
viz.go(viz.FULLSCREEN)

groupL = viz.addGroup(pos=[0,1.0,5.0])

boxL = vizshape.addBox(parent=groupL,color=viz.BLUE,pos=(-2.0,0,0))
sphereL = vizshape.addSphere(parent=groupL,color=viz.GREEN,pos=(1.0,0,0))

groupL.alpha(.5)