I apologise for very late reply.
Thank you for explanation regarding contamination by the coloured canvas cover.
First of all, I like to get better understanding by asking below questions.
1. Contamination due to blue pigment because of wet canvass sheet caused by rain or broken off pieces from canvass?
2. You mentioned some shippers already agreed to switch to clear plastic whereas others still refuse.
My concern changing clear plastic can solve the colour issue but what type of plastic material they use?
Please check the melting point of the plastic not > 109C otherwise any broken piece difficult to spot if stick to rubber.
As far as I know tarpaulin or canvass sheet can be made from PE , PVC with high melting point between 140 – 160 C, Cotton canvass sheet (Waterproof type)
Coloured material more advisable in case of broken pieces can be spotted and remove.
Suggestions:
As you mentioned these problems started only in June this year, so try to identify any 3M change. E.g. the quality of pigment (colour) added during processing of poor quality etc.?? because shippers pay lesser ??
Are there some shippers whose coloured tarpaulin cover does not give this contamination issue? If yes find out who are their suppliers and any difference in raw materials including pigment used.