The mourners are in black: the black of grief, the black of ashes and broken hearts. The black of soil from a fresh-dug grave. The black of the unknown, of the emptiness and the void. The black of a space where someone once was.
Necromancers wear red.
The red of blood, the red of life, the red of love and passion and memory and pain. The red of hearts, and eyes swollen with crying. The red of poppies in fields and roses on wreaths. The red of wine given in offering to fortify the soul.
In Egypt, black was the colour of delta mud, rich and ready for planting. Black was life. Red was the sun looking down in fury and the sterile dunes of the desert stretching before you. It was red that stood for death.
Mourners are entrenched in, and focused upon, life. When that is gone, all they can see is the pain left by its absence. Deeper than a well, wider than a church door: the endless void-like loss of a life and love that will never return.
Necromancers see life in a broader definition. You are in part the sum of all those who came before you and those who follow will carry a little of you onward in turn. The body is gone, but the memory, the influence, the soul (and sometimes the ghost) is not.
And that is why at wakes and funerals - with the utmost respect - necromancers wear red.