(This post is an excerpt from a this choral music review.)
The presence of accidentals does not make a passage is chromatic. Chromatic passages occur specifically when you change the accidental of the same pitch. So for instance, in the key of C:
- E-F#-G#-A is a diatonic line, even though it has accidentals, because the accidentals still create a segment of a diatonic scale. You never hear, say, F and F# in juxtaposition.
- E-F#-G and E-F-G are a diatonic lines for the same reasons.
- E-F-F#-G is a chromatic line, because the accidental alters the previous pitch and because diatonic scales never feature consecutive half steps. The half steps in diatonic scales always follow and proceed whole steps.
Thus in the key of E-flat, the line C-A-Ab (mm. 76-77 in the tenors) is chromatic.
It should also go without saying that chromaticism is also not necessarily harder, stranger, or wrong. It’s simply a different relationship between pitches than a diatonic one.