Watch the Dark Rainbows online guitar lesson by Chris Buono from 50 Metal Guitar Licks You MUST Know
When trying to get a great metal riff going a good idea is to of course utilize the prerequisite power chord sound, but there's more to it than that. A simple way to get a progression of power chords to sound interesting is to inject some bass notes in between the changes to break it up and get some movement happening. An example worthy of metal hall of fame status is the "Rainbow in the Dark" riff played by Vivian Campell (pre-Def Leppard Viv, that is) from Dio's Holy Diver album. This Dm version has similar attirbutes including the toggling of half-steps between the bVI Bb5 chord and the open A string as well as a cool walk-up idea to get back to the top.