https://mods.factorio.com/mod/deadlock-beltboxes-loaders
IIRC, prior to version 2.0.0, this actually colored trains filled with the compressed items, but now they all appear for me as RGB: 208/209/199.
Ideally, they should be given the same colors as their uncompressed counterparts.
IIRC all compressed stacks have an ID like: "deadlock-stack-<vanilla name>"