err...to my often-dodgy memory, i recollect it's thiocyanide (SCN-). Basically an addition of SCN- anions to Fe3+ will give a deep blood-red [FeSCN]2- complex. Addition of SCN- to Fe2+ won't do anything. This is basically to tell between Fe2+ and Fe3+
The addition of the S to the ion molecule makes it not so deadly, I think. The entire properties of the ion changes... and yes it forms the deep, blood red colour with Fe3+.