A friend and I redyed the blue carpet in his 250 SL. It was a royal pain to do, as you had to have hot (as in close to boiling water) to dissolve the dye, and then quickly spray it on the carpet. It has held up pretty well. He is now replacing his carpets as there were a lot of wear spots, the dye was a good interim solution for him. For carpet in good condition it would probably work well. I will check with him on the source.
The difficulty may be on finding the right dye. His carpet had faded mostly to gray/white. You will need to consider how the colors will interact to get back to the brown.
We did take all the carpet out of the car. The only item I am not sure we removed is the panel around the storage area.