finally find from other posts, the octane node "color vertex attribute" can help to transfer the color attributes from geo node to octane material nodes.
though, still there is bug,
when the number of stored attribute is more than 2 or three, the behavior of "Color vertex attribute" become strange:
1, when adding first stored color attribute "colour" both the previous good one "colorAtt"(which is from geo node group output) and the new attribute works, which is good.
when add one more stored attribute that is the blue channel (attribute "b") of the "colour", suprisingly the "colorAtt" changed its color, which is not it supposed to be.
though at this time the attribute "b" still works fine.
further more I add one more attribute the green channel and the red channel, it seems the "r" and "g" will work as is should be but the "b" does not.
here is the final .blender file: