So I dyed them with a mixture of Procion MX dyes. They are strong and will stain everything in sight.
I put the mats in garbage bags and waited 24 hours. Then rinse, rinse, rinse... and dry.
Here's the driver's side mat. Half with dye sprayed, half not.

Here it is wet after a rinse or two.

Here it is today after multiple rinsings... with it's buddy. In person, these look very dark. The lighting in this pic makes them look faded. The aubergine color has really popped. though.


Overall, I'm pretty happy with it.
-Rusty