I usually use black RIT dye in the washing machine, with a cup of salt, and it works great on cotton and rayon. I do this to freshen up black pieces that
are in great shape, but faded. I will usually use 1-2 liquid bottles for 4-5 tops, and adjust the size of the load accordingly. After dying, I reset the load to extra-large, and run an empty wash with a cup of bleach and some detergent, to clean the dye out of the machine. I wipe any splashes with an old rag and dishsoap, and toss that into my bleach wash.
For a viscose dress, I did some research online, and used black Dylon - but I did it in the sink with hot tap water (using the tutorial I found online). I had mixed results - some places the dye took and was beautiful, and other places... not so much. I will eventually try this again, with RIT or Dylon, in a small-size washing machine load, set on gentle.
I bought Dylon at JoAnn's Fabrics, and they were having a sale. It came out to a little more than RIT dye at the supermarket. I know there are also coupons you can get online, for your smartphone.
As to the smell, if you do it in the washing machine, the smell isn't too bad. I had to dye an oversize powder puff pink for a prop, and I did that on the stove top, and it was kind of stinky. I cranked the AC down and put the ventilation fan on. Same for the sink-dying.
And wear gloves!