This paper describes a virtual prototyping (VP) system which is a part of an open source software package for an additive manufacturing (AM) process under development. The VP system facilitates the product development by uniting the AM process and virtual reality in order to produce digital prototypes. Moreover, it combines particle based and layer based processes by including powder-like particles as its basic material. These particles are used as color codes in the VP system. This coding enables obtaining basic building blocks in homogeneous state or in heterogeneous state by mixing with other particles. These blocks or bricks are collated side by side to obtain the heterogeneous material property all over the solid body. The thin layers obtained by these bricks are then subsequently stacked up to fabricate a virtual prototype. Construction of multiple material prototypes is possible due to selective-additive nature of this process. The effectiveness of the proposed system is demonstrated by processing a model of The Maiden's Tower. © (2012) Trans Tech Publications.