I had seen this wonderful tutorial when searching for worsted weight rainbow yarn, and considered trying, but didn't want to buy guar gum OR have the colors bleed. I tried to think what would work that I already have, and I had arrowroot powder. It ended up working great, so I'll share what I did.
After skeining the yarn (worsted weight wool and alpaca blend) in a 2 yard skein, I soaked in vinegar and water for at least an hour.
I took 2 cups of vinegar and 1 Tbsp of arrowroot powder and heated in a pan over medium low heat, whisking often, until it the early signs of bubbling. Then I turned off the heat and let it cool to a touchable temperature.
I poured or scooped it into paper cups.
I mixed in the food coloring drops and blended until I liked the colors.
I squeezed the vinegar water out of the yarn and laid it on plastic wrap.
I pinched the cup for a spout to pour out little bits at a time and worked the gel in carefully with my fingers.
My grey blend split and wasn't enough, so I fingerpainted the remaining colors in until satisfied.
I wrapped the plastic toward the middle to separate the loop and put on a plate. I microwaved according to that tutorial's directions, and once cool I washed it out gently in warm soapy water and rinsed in clear warm water. No dye bled! Now it's hanging to dry. :)