How to let Rhino show the true colour for Stable Diffusion's segmentation

Hi all, I am trying to make a Rhino template which can be applied to the Stable Diffusion’s ControlNet segmentation. But I am not sure how to let Rhino to show the true colour – no matter I use shaped mode or render mode, it will have a shadow effect on the Rhino objects(as shown)


You can see I have applied the RGB(255,6,82) colour on to the Rhino object. But the top, left and right face are showing different colour. How can I make them to be a true RGB(255,6,82) colour?
Many thanks