I am the same - prefer natural colors but open to blue and green for a pop of color.
I actually went for the same sizes as you for my first B's and K's. I have three B30 (black ghw, black shw, and etoupe ghw (although I would very much prefer etain as I am not a fan of white stitching)) and a K28 sellier noir before I decided to go for the smaller sizes B's and K's.
I got a K25 retourne (purposely asked for retourne as I think sz 25 will be too small in a sellier construct) in noir ghw.
My next bags I would go for a B25 (or two) in either deep blue, malachite, or Rouge H. A bit more fun in color and should be OK in a sz 25 bag. Maybe you can consider these colors (or even the sizes?)