Is 'natural' colored filament equally brittle compared to white filament?
No
Natural colored filament has no pigments added, the ivory off-white color is the natural color of the mixture in the filament.
Also note, those white filaments can have much less than 50 % volume percentage of pigment, depending on the exact hue of white. The starkest cold whites might make that, warmer white can work with a lesser amount. Pigments are part of the filler materials.  The other fillers make the filament behave in the designed way and can make up a considerable amount of the filament. Usually, less than 40 % are fillers compared to the PLA.