Some buff colored wall surfaces are marred by horizontal, slightly wavy lines. The concrete above and below appears to be uniform, so there couldn't have been any differences in the mixes. The lines appear to mark the tops of successive lifts. How can these lines be avoided?
What you describe are lift lines. Active use of the vibrator and deep penetration into the preceding lift (at least 6 inches) is the key to their elimination. If vertical construction joints are placed reasonably close together the horizontal distance spanned by each lift will be smaller and problems associated with long periods of time between placements of two successive lifts will be minimized.