Can you post a working script that reproduces this specific problem? I tried the following script and it works for me. It loads 2 OTF nodes underneath a group node. When the spacebar is pressed, it will set the color of the group node to red, which gets applied to the OTF nodes.
Code:
import vizshape
viz.go()
group = viz.addGroup(pos=(0,1,10))
box = vizshape.addBox(parent=group,color=viz.BLUE,pos=(-1,0,0))
sphere = vizshape.addSphere(parent=group,color=viz.GREEN,pos=(1,0,0))
vizact.onkeydown(' ',group.color,viz.RED)