Hi Paul,
I think that the smell is sometimes a combination of damp wool and damp backing.
Jute has its' own damp smell. Also the bonding agents (mixtures of latex, oil and filler) have varying formulations which can result in different odours.
The colour of the dye should not have an effect on the odour, since the dye sites are closed during manufacture, but can be opened by the cleaner by use of excess heat.