After having purchased both the bright orange and saddle brown it’s become evident that out of the box at least there is a difference in the leather used between colours.
The bright orange leather case is much more textured than the saddle brown, which is smooth. This makes it slippery, when holding on the sides it’s almost unusable. Even using within the 15 day return period I’ve found that the grip doesn’t increase. I know it will over a long period but I can’t wait, I’ll most probably drop it.
So I’m returning the saddle brown and sticking with my bright orange, when I use a case that is. I already have so many scratches on my iPhone X that I’m less worried about dropping it. I have Apple Care + just in case.
My advice if you want a leather case from Apple is to try before you buy in store. They don’t mind you opening them up and having a go, you will find the grip changes depending on the colour you choose.