If you have something like Adobe Premiere or perhaps even Photoshop: 1 render your object in .tif format with each color so now you have two different .tif files or two different animation sequences. 2. bring in your .tifs as separate layers 3. in Photoshop smoothely change between the colors by varying the transparency of the top layer. save out along the way if you want to make a .gif sequence. 4. if you're in Premiere you can cross fade between the layers. the duration of the crossfade is easily set. just an idea. But if you are asking how do I do it in Poser? The materials aren't keyframable nor is the transparency. sorry.