Wiltons will often break up giving you the different color changes. Many of us like the way it makes the yarn knit it up, but as always, knitter's preference. Purple is notorious for it and creates some gorgeous violet, blue, purple, red stuff.
Sometimes you can combat the breaking up by dissolving the dye in water very well. Make sure it's completely dissolved. Put it in the dye pot and stir will, heat to almost boiling. Add soaked yarn and mix so it's all submerged. Then stir in the vinegar.
The dyes may not break up as fast before striking and you'll get orange, or they make break up less so you'll get less variation in the finished yarn.
Dyeing can change, somewhat, every time you do it depending on the circumstances...with measuring everything you can get closer, but it's hard, without being able to insure the pH of your water, and that the vinegar is exactly the same strength, etc., of getting the exact same product each time.
That's the fun of it. Aiming for one thing and sometimes getting something close but just as beautiful as what you'd imagined.
If you're totally unhappy with the yarn you have, overdyeing with a primary color may help. The ones that break apart are the secondary and tertiary colors because they are a blend of primary colors: yellow+red= orange, blue+red=purple, etc.