Watching footage of the Orion test launch on Dec 5th, one can see orange flames rise up from the base of the rocket in the few seconds before t=0 and before the engines appear to fully "fire up".

Nothing like it is visible on Space Shuttle launch footage.

What is happening in the launch sequence to produce these effects?

A description of the Saturn V mentions that ignition was accomplished by "bursting a hypergolic cartridge". Does the Delta IV heavy do something similar?