How do Z and atoll X-ray binaries differ?

Low-mass X-ray binaries containing weakly magnetized neutron stars may be divided into two classes, Z and atoll sources, based upon correlations between their X-ray timing properties and the patterns that they trace in plots of two X-ray colors. In this Letter we examine color-color diagrams of eight atoll sources and four Z sources using data from the Rossi X-Ray Timing Explorer. The 5 year span of data we have examined is significantly longer than the spans of previous studies. We find that the previous clear distinction between color-color diagrams from atoll and Z sources is an artifact of incomplete sampling, as those atoll sources that are sampled over a wide dynamic range in intensity (F-max/F-min greater than or similar to 80) trace three-branched color-color patterns similar to the tracks for which Z sources are named. However, atoll sources trace this pattern over a larger range of luminosity and on much longer timescales than do Z sources and exhibit much harder spectra when they are faint, which argues against any simple unification scheme for the two classes of source.