power chords are the root and the fifth of a "normal chord" so your key will be the root of the chord which seems to be the base of your riff.
For exemple in metallica's enter sandman, the key is E, since this tone attracts the riff to come back to it...
Chromatics are notes that are between actual scales notes (to play a chromatic scale, you just have to play every note on every fret on your neck)
when used tastefully, chromatics add nice "colors" to your solos (chroma= color in greek)