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Color and spectral observations are important for understanding the taxonomy, composition, formation conditions and evolutionary history of Trans-Neptunian Objects (TNOs). Colors or spectra do not, however, uniquely constrain the compositions of these objects, as we demonstrate using simple spectral mixing models. We show that thermal flux observations at 70 μm (or any wavelength near or beyond the peak of the object's emission) are much less sensitive to details of how the materials are combined. Coupling reflectance and thermal observations of TNOs will help remove much of the ambiguity intrinsic to color and spectral data.