Halo 4 - How to find the Red vs Viola easter egg
In the Spartan OPS mode of Halo 4 there are several hidden easter eggs concerning the "Red Vs Blue" series (in English it is Red Vs Blue).
Finding one of these easter eggs will unlock a game achievement
Reds against purple - Find one of the RvB Easter Eggs in Spartan Ops.
In the video below you can see one of these easter eggs, in Chapter 5: Core of Spartan OPS. Shoot the crate as shown in the cutscene to start the sequence.