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We have obtained milliarcsecond-scale spectral index distributions for asample of 190 extragalactic radio jets through the MOJAVE project. The sourceswere observed in 2006 at 8.1, 8.4, 12.1, and 15.4 GHz, and we study thefour-frequency spectrum in individual jet features. We performed detailedsimulations to study the effects of image alignment and (u,v)-plane coverage onthe spectral index maps to verify our results. The core spectral indices are onaverage flat, while the jet spectrum is in general steep with a mean index of-1.04. A simple power-law fit is often inadequate for the core regions, asexpected if the cores are partially self-absorbed. The overall jet spectrumsteepens at a rate of about -0.001 to -0.004 per deprojected parsec when movingfurther out from the core with flat spectrum radio quasars having significantlysteeper spectra than the BL Lac objects. The spectrum in both types of objectsflattens at the locations of the jet components indicating particleacceleration or density enhancements along the jet. The mean spectral index atthe component locations of -0.81 corresponds to a power-law index of ~2.6 forthe electron energy distribution. We find a significant trend that jetcomponents with linear polarization parallel to the jet (magnetic fieldperpendicular to the jet) have flatter spectra, as expected for transverseshocks. Compared to quasars, BL Lacs have more jet components withperpendicular magnetic field alignment, which may explain their generallyflatter spectra. The overall steepening of the spectra with distance can beexplained with radiative losses if the jets are collimating or with theevolution of the high-energy cutoff in the electron spectrum if the jets areconical. This interpretation is supported by a significant correlation with theage of the component and the spectral index, with older components havingsteeper spectra. (abridged)

MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. XI. Spectral distributions