I agree with Seton and Asa. Ardoise has a cooler bluish undertone while graphite is warmer with a hint of brown.
Graphite in Croc is amazing!! I would think the undertones are more pronounced in croc than in leather. I guess that will always be the case with exotics. I wonder if it is the saturation of color or the skin itself thats makes the colours look different?:wondering
I have never seen Ardoise in real life, but I have a Graphite Birkin in Clemence and I would indeed describe the colour as warm grey. Depending on the light and the clothes that you wear it blends with the environment.