"Red" measures
The primary observation is that there is a two-beat pattern moving down in whole steps. So, following your lead from C#7, the chord two beats later would be B7, and then, again two beats later, A7.
From there, a variety of options exist.
Following the C#7, we have a delayed Gx, a Dx, which clearly sounds like a "leading" tone, and a D#. The D# and Gx (plus the A# passing tone in the C#7 note grouping), give a D# chord. This pattern of intervals is continued every other beat, giving C#7 D# B7 C# A7 B
.
The final note of the C#7 beat can be heard as a suspension into the next measure, resolving by half-step. The final note of that beat-after-C#7 can be heard as an anticipation of the following chord (B7). This allows for a simple progression of dominant seventh chords, descending by half step: C#7 Bx7 (C7) B7 A#7 A7 G#7
.
Taking advantage of the fact that each chordal seventh moves down (the expected resolution in a typical V7 - I cadence; e.g., in the C#7 chord, the B moves down to A#), we could change chords every half-beat to capture that sense of V-I resolution: C#7 F#7 C7 F7 B7 E7 A7 D7 G#7 C#7
, which perfectly sets of the final two beats as F#7
and B7
.
"Blue" measures
What catches my ear in this passages is the half-step moves into each half beat. I hear this passage, starting on beat 3, as D# G#m C# F#m B Em A# D#m
. This makes the last two beats of the measure G#, which leads to C#m and a two-measure final cadence in C# minor.
If you want to get fancy, could could make each beat of the sequence:
1/2 beat: D#
1/4 beat: G#m
1/4 beat: G# half diminished (this accommodates the D natural)
1/2 beat: C#
1/4 beat: F#m
1/4 beat: F# half diminished (accommodating the C natural)
etc.