tldr: The song is primarily in C major, mixolydian mode, but briefly changes keys to G major, mixolydian mode, in the middle of the verses. There is a G major V chord between the intro and verse that, although not native to mixolydian, acts like a secondary dominant to help establish the C chord as tonic. The solo just follows the chords and key changes.
Candyman full analysis
The song opens with a downward progression by fifths from Bb to F to C (see discussion of a circle of fifths progression in Hey Joe). The chords are all from the key of F major, but the tonic is clearly C, making it C mixolydian, with the Bb serving as a bVII chord. So that’s bVII – IV – I.
The C tonic is then reinforced by a transition G chord, which is the V chord of C major (and not diatonic to F major), and which resolves to C again to open the verses. The v chord of C mixolydian would normally be Gm, but the G doesn’t mean we are switching to C natural major. (In fact, the song goes right back into mixolydian after this.) Instead, the G serves as a secondary dominant “V of I” chord, borrowed for its functional leading quality towards the C chord. This both sets up the ensuing C chord and confirms its role as tonic.
The verse then goes right back to mixolydian with a Gm v chord, in a I – v – IV progression of C – Gm – F.
The F then transitions to a G chord. This time, though, the G chord neither is temporary nor resolves to a C chord in V -> I fashion, indicating that there is a key change. One could argue that we are switching to C major, but based on how much this part of the song centers on G, I think the better explanation is that the G is established as a new tonic, in mixolydian mode. (C major is the parent key of G mixolydian, so they share the same notes and are distinguished only by the tonal center being C or G, respectively). There is a G – Dm back-and-forth that is therefore a mixolydian I – v progression, providing a counterpoint to the I – v that we had earlier in C mixolydian. And then an F – G that is bVII – I.
From there we transition right back to C major, mixolydian mode, returning to the opening Bb – F – C progression to start the chorus. Although there is no pivot chord shared by both keys to ease the move back into C major, the Bb chord isn’t abrupt from G mixolydian. As a bIII from the perspective of G, the Bb chord (Bb D F) shares two notes with the diatonic iii diminish chord of G mixolydian, Bdim (B D F) and is an easy substitute. There is also an Am or Am7 added in here, which is just the vi chord of C mixolydian (and C major too, for that matter).
The solo just follows the chords and key changes, C mixolydian to G mixolydian.
- Major V chord in mixolydian helps establish the tonic
- Circle of fifths progression in mixolydian of bVII – IV – I.
- Substituting a bIII for iii dim in mixolydian mode to change to a key containing that bIII chord.