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The physical origin of the much studied optical/UV ``Big Bump'' in quasars is unknown. Even its suggestive connections with ``soft X-ray excesses'' is uncertain. We have investigated several models for the blue bump and the soft X-ray excess: (a) free-free emission from an optically thin plasma (see Barvainis 1993 for a review), (b) optically thick thermal emission from the innermost regions of an accretion disk in both Schwarzschild and Kerr geometries (e.g., Shields 1978, Malkan \& Sargent 1982, Czerny \& Elvis 1987, Sun \& Malkan 1989, Laor 1990, Fiore, $et al.$ 1994). We present the locus of each model in optical/UV/soft X-ray color-color diagrams. The colors were optimized to be indicators of particular spectral components and they provide stronger discriminants than the standard UBV diagrams. These diagrams clarify the limitations of both models in their pure form. Addition of extra components is needed and we show how the simplest possible addition (an underlying power law) brings disk models into agreement with the data. We compare our results with the sample of 47 low z quasars for which we have collected observations over a wide range of frequencies (Elvis $et al.$ 1994).
