nze -
You can use Marshall's dyes just like you used Spottone.
In fact, Marshall's are a bit more convenient because they come prepackaged in bottles with droppers.
I put together a drop mixture on a white plastic palate, then using a damp brush, I make a set of smears on the palate, and let everything dry. Then, when I have to spot a print, I use a barely damp brush to pick up a bit from the smear that is slightly lighter than the area surrounding the white spot on the print. Used that way, the dyes last forever, and its much easier to match tones.