You could use an AttribPromote node. Set ‘Original Name’ to the attribute that you want to take the average of, ‘Original Class’ to ‘Primitive’, and ‘New Class’ to ‘Detail’. I would also check ‘Change New Name’ and put something like ‘AverageColor’ and then also uncheck ‘Delete Original’ unless you want to delete the color. Leave ‘Promotion Method’ to ‘Average’. You will then have ‘AverageColor’ as a detail attribute and it will be the average of all the prims' colors.
If you want to average just a specific group, I would isolate the group with a blast node and ‘Delete Non Selected’ checked, do the average, and then merge that result with the result of a blast node with ‘Delete Non Selected’ unchecked. Make sense?