You have to understand that 'color' doesn't really exist. Light is just another part of the electromagnetic spectrum like gamma rays, x-rays, microwaves and radio waves. These are all just photons at different wavelengths or energy levels. You brain takes the signal it gets from cells that are sensitive to certain frequencies and creates the hallucination that we call vision. Purple is just what your brain tells you that certain frequency is.

The exact composition of the liquid will make a difference, but since softdrinks are mostly water I would imagine that it would be very close to 1.5 kg. The difference might be so slight that your scale can not go to that granular a level. The mass of the bottle will make a difference, as will the slight difference in fill level.

This is not true. The stomach is also mostly on the left side, with the liver mostly on the right.