Select level XLI (Docks). You need to have 4 different colored flames at once. You start with your basic flame. Kill the red, green and blue ants to obtain their color. Make sure all colors burn at the same time otherwise the achievement will not unlock. It might take you a few times to figure out the path to take to have all 4 colors burn.
Edit 10 October 2011: Hint as an answer on SwamiMacGyver
I tried it a few times and when timing is perfect those work:
* Hold your phone like the level start. Green and Blue ant go left, red ant goes up or down (does not matter). Burn the rope where the ropes crosses (rope of the green and red ant). Hold your phone a little bit diagonal so the flame burns the rope of the green ant and the red ant. Make sure that when you cross the rope of the blue ant, you let it burn to. With a little bit of luck, you have a standard, green, blue and red flame.
* Hold your phone how the level starts. Green ants must go all to the left, let it turn to the right. Blue ants will probably going to the right and is half way. Red ant must preferable go up.
Burn the crossing of the ropes of the Green and Red ant. Hold your phone diagonally so the ropes of the Green and Red ant burns. Turn your phone so the rope of the Blue ants burns. Hopefully the Red ant is not there yet because your standard colour needs to burn the parallel rope of the blue flame. With luck you have:
a green flame on the Green ants rope
a blue flame on the Blue ants rope
a red flame on the Red ants rope
a standard flame on the parallel (of the Blue ants rope) rope.
Edit 23 October 2011: Credit to StoneyMcChronic
: Achievement unlocks upon level completion.
Edit 14 August 2012: According to barneyrobble1
you can fail the level and the achievement still unlocks.