What color are the buckets? Does the original product come in a bag inside the bucket?
Most plastic colorants are not FDA approved for direct food contact. While HDPE is safe, the low pH of wort/beer could leech some chemicals out of the colorant in the plastic.
Most, if not all fermenter buckets I have seen are white which is colored with titanium dioxide (pulverized rock) which is food safe.
I would not use them if they are any color but white as you never know what is in the colorant formulation.