I've been wondering the same thing. Trying to turn a yellow Salt purple.
I've tried different combinations of purple, blue, and scarlet.
I tested on yellow nylon zip ties.
Using 2Tbsp of color to 4C water, near boiling, with a drop of Woolite, 20 shakes of salt, and about 1/2 tsp vinegar in the solution,
These are some of my results:
Left to right:
Original, 2T purple - 2 minutes; 2T purple for 1.5 min & 1T purple + 1T blue for 1/2 min; 1T/ea royal blue and purple - 2 min; 1T royal blue, 1/2T purple, 1/2T scarlet - 2 min; 2T Royal blue + 2T purple - 5 min.
I know they didn't come out great in my photo.
In sunlight, they look:
Yellow, reddish brown, forest green, dark green, OD green, and black.
I think with a yellow base, your best bet is to go for a green or an orange.
Unless someone can give a detailed description of how to get purple from yellow, I'm going to leave my Salt alone.