If you are flying around your model in the 3D window with your client looking over your shoulder then OpenGL is a reasonable choice. Internal engine is slow and has no textures, but does have vectorial fills. Therefore the easiest way to change the colour of your siding is to duplicate the image used and adjust the colour in Photoshop or other image editing software. Easy for plain colours, harder for textured materials like wood grain, but that would depend on your level of expertise. Mine is not great. Assign the new image to your surface (must be in an available loaded Library).
Of course you could search the internet for a suitably coloured image and replace your surface texture with that. There are plenty out there.
If you have, say, 3 siding choices in your surfaces (e.g. light, medium and dark) you can have a whole range of images available in various colours to assign to these options. Then if you want to change the colour of your light siding, just replace the image and all elements using that surface material will change. So you don't have to have materials set up for all colour options initially.
