I have used PET plastic for years and years for both B&W and color developers (and all the other chemicals). I have had NO deleterious effects ever as long as the bottle is filled TO THE VERY RIM. (And I do not worry about keeping them in the dark even though they are clear.)

This 'filled to the rim' caveat is the annoying part because I have to use glass marbles to take up the slack. However, there are some PET plastics that can be slightly squeezed, such as Pepsi soda bottles that are long and narrow and have largely clear sides without indentations. But these are usually about 500ml. How I would like smaller bottles that could be likewise squeezed as it is a hassle to keep adding those marbles. For very small quantities, I use 50ml liquor bottles but, again, marbles (tiny ones from arts and crafts stores) are needed. - David Lyga