it's easy to make a greyscale texture in poser 6 -- just slap an hsv node in front of the image map for the iris, and set the saturation value to 0 (connect the iris's diffuse color channel to a math -> hsv node, connect the hsv node's color channel to the texture map). that alone won't make the texture interesting, of course. i tend to do that sort of work in photoshop where i can modify the curves of the individual colour channels and get more interesting contrasts. flatness can also be combatted by making the eye textures shiny, wet, with nice specular highlights. i am on a different machine without my tutorial links, but i am sure somebody else will jump right in.