First and foremost, read the manual (as crappy as it is). From the Materials, change the necessary material zones to white. Otherwise, open the horse template in Photoshop, paint the entire template white, "Save as..." with a different name (crucially important), load this new texture from the Materials panel, apply the texture to the necessary material zones. Unless you paint the various corresponding areas of the template a different color (i.e., the hooves, mane, tail, etc) they will all be white as well. IIRC, the texture is affected by the underlying Material zone colors, so applying the texture to a base white is a good starting point. To view the textured model in the studio/stage window, select the rightmost sphere (looks like a striped billiard ball) of the Document display (the series of spheres beneath the main window). If you've already tried these steps, consider that the lighting may make the horse 'look' greyish.