Cotton candy (lighter pink) in the left hand (which puts it underneath and thus the dominant yarn).
About the can’t-leave-well-enough-alone thumb: I inserted the waste yarn as written in the pattern and finished knitting the body of the mitten. When it was time for the thumb, I removed the waste yarn and put the live stitches on two double-pointed needles. Then I attached the yarn again by picking up two stitches between the needles (kind of the side of the thumb), knit across the front of the thumb in pattern, picked up four stitches on the other side of the thumb, knit across the underside of the thumb alternating colors every stitch, and then picked up two more stitches at the end of the round. Overall I added two stitches on the left and right of each side (so 8 total) to continue in my established thumb pattern. When the time came to decrease I did it the same way as the mitten top (ssk on the left, k2tog on the right) using the darker color for each decrease stitch until I had 8 stitches on each needle, and then I grafted the top. On the last round I had to k2tog in the middle of the underside to get an equal number of stitches on both needles (this is due to the way stitches are picked up from waste-yarn-held live stitches - you get more on one side).
29g cotton candy left 27g fuchsia left
mittens with ends weigh 45g = 207.9 yards