Functionally graded materials (FGMs) are heterogeneous materials engineered to vary material composition across the volume of an object. Controlled mixture and deposition of each material through a manufactured part can ultimately allow for specific material properties defined in different regions of a structure. While such structures are traditionally difficult to manufacture, additive manufacturing processes, such as directed energy deposition, material jetting, and material extrusion, have recently increased the manufacturability of FGMs. However, the existing digital design workflow lacks the ability to accurately mix and assign multiple materials to a given volume, especially in the case of toolpath dependent deposition processes like filament-based material extrusion. In this paper, we will address this limitation by using a voxel-based representation approach, where material values are assigned across a pixel grid on each geometry slice before converting to toolpath information for manufacturing. This approach allows for creation of structures with increased material complexity decoupled from the external geometry of the design space, an approach not yet demonstrated in the existing literature. By using a dual-feed, single melt-pool extrusion nozzle system, this research demonstrates the ability to accurately recreate mathematically derived gradients while establishing a digital workflow capable of integrating with the material extrusion AM process.