  The long-term optical and infrared color variability of blazars has been
investigated with the SMARTS monitoring data. The sample in this study consists
of 49 flat spectrum radio quasars and 22 BL Lac objects. The fractional
variability amplitudes of each source have been calculated in both optical R
band and infrared J band. Overall, the variability amplitudes of FSRQs are
larger than those of BL Lac objects. The results also suggest that the
variability amplitude of most FSRQs is larger at lower energy band (J band)
than at higher one (R band), while the variability amplitude of BL Lacs are
larger at higher energy band. Two types of blazars both display color variation
along with the variability in brightness. However, they show different
variation behaviors in general. With the whole data set, 35 FSRQs exhibit
redder-when-brighter trends, and 11 FSRQs exhibit opposite trends; 11 BL Lacs
follow bluer-when-brighter trends, and 7 BL Lacs follow opposite trends. The
examination in detail shows that there are 10 blazars showing
redder-when-brighter trend in their low state, and bluer-when-brighter or
stable-when-brighter trends in their high state. Some more complicated color
behaviors have also been detected in several blazars. The non-thermal jet
emission and the thermal emission from the accretion disc are employed to
explain the observed color behaviors.
