I think we can take it as a given that the blue and black models will show scratches more than natural or white, they probably are no easier to scratch though.
The bigger question is how easily does this coating scratch full stop? We have had coloured aluminium on phone sides for years and yes if you do scratch them it shows, but it is not super easy to do so. If the titanium iPhones are on that level then I am ok with it. If on the other hand they scratch up like an old iPod classic or that horrible glossy black phone they released back in the day then that is a different matter!
I am not seeing any indication that they scratch easily though, just that if the do scratch it will show more?