@cognitdiss wants the CG of an assembly of multiple objects with the individual objects made with different density materials. Presumably each component is a separate “solid” in Rhino. For a object with uniform density the CG is at the volume centroid of the object. However for an assembly of objects of varying density the CG is not at the composite volume centroid of the objects.
Similar requests have been made previously. Rhino does not have this ability. There are plug-ins which include the ability to track the density/weight of individual components and calculate the composite CG location. Orca3D, a plug-in for boat design, is one example.
An alternative is to use a spreadsheet and input the volume, centroid coordinates and density of each component, and have the spreadsheet calculate the composite CG. The spread sheet can also calculate the weight of each component and the total weight. Attached is an example of a simple Excel spreadsheet. CG Calc DC01.zip (5.6 KB) (Uploaded as a .zip file because this website does not allow direct uploading of .xls files.)