I agree they need to be darker, as you said anthacrite/gunmetal, and as you may have noticed I'm going for the dark theme (black head lamps, smoked front fogs, dark rear lamps).
I chose tyre sizes per LOC wheel size guide, and using a tyre size calculator, 225/40/18 have nearly the exact same diameter as 255/35/18. I realise they are 255/40/18 on the IS250, and they are ok on your car, but as mine is lowered I chose to be more conservative with the size.
http://www.lexusownersclub.co.uk/forum/ind...p?showtopic=637