Cuuuuuute! Reminds me of the types of games I used to play on Club Penguin. It'd be interesting to see what adding more colors might do to the difficulty, like Black/White. I'd imagine that would make it a little more challenging, since I actually didn't fail a single mission!
My tactic ended up being to grab all of the colors except the ones I wanted at that exact moment, then I'd swoop up the ones I needed. That strategy might become less efficient when there are more colors that you need to grab!