John:
From your screen shot it looks like there is a texture applied to the material (Surface), you can check this by opening the Surface Settings (was Materials Settings before AC17), selecting the stucco material and looking at the Texture panel in the dialog box. You can either remove the texture from the Surface, or select another texture that is whiter, or increase the Ambient and Diffuse values in the Reflection part of the Exposure to Light panel of this dialog box.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David