This challenge was designed in an effort to show the participants how to play pixelflut, this was an attempt to both work with the space we're in as well as motivate conversations.
The challenge description reads "use your red ff0000 flare gun to communicate with someone on the pixelflut canvas"
So, carefully examining the pixel on the canvas which you set to red, setting it to red by sending PX N N ff0000, and reading back the same pixel by sending PX N N, you can see that for each second that passes, the colour of the pixel changes in the format of ff | NN | ff.
Concatenating all of those changed pixels over the course of a couple of seconds, yields the flag: DHCP{MY_FL4R3GUN_1S_V3RY_BR1GHT}