Painting on textures from Octane material is limited only to white color. There is no way to change color.
Not only painting but even fill bitnmap doesn't work with other colors.
Probably because Octane channels are not connected to corresponding ones in C4D, they are called "custom" :
Paint Wizard Setup doesn't work too. It don't know where, to which Octane material channel connect created texture.
All other C4D materials works fine, node based and Redshift ones too.