If you're doing intarsia (which by the size of the ducks in the picture, this is what they want), then you need to have multiple balls of blue and one of yellow.
Start off the row knitting with the blue, switch to yellow, then switch to the next ball of blue. When you come back in the other direction, you do the same thing, knit with blue, pick up the yellow from the previous row, then pick up the next ball of blue.
I know I posted it before, but here's a picture of the chickie I did on a baby snuggle I'm making. I ended up with about 6 different balls of yarn at one point. And I ended up with a hole or two that will need to be tightened when weaving.