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I got some green ones thrown in, so I chucked them out. (bio-degradable) ;)
The blue trays quickly get brittle and can snap in your hands when loaded with pots. Try to get the black ones that come from Spain as they are much stronger. Also they have enclosed hand holes at each end which allows two trays to be positioned end to end and easily carried with one hand. This permits 4 loaded trays to be carried with ease.