  We have obtained milliarcsecond-scale spectral index distributions for a
sample of 190 extragalactic radio jets through the MOJAVE project. The sources
were observed in 2006 at 8.1, 8.4, 12.1, and 15.4 GHz, and we study the
four-frequency spectrum in individual jet features. We performed detailed
simulations to study the effects of image alignment and (u,v)-plane coverage on
the spectral index maps to verify our results. The core spectral indices are on
average 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, as
expected if the cores are partially self-absorbed. The overall jet spectrum
steepens at a rate of about -0.001 to -0.004 per deprojected parsec when moving
further out from the core with flat spectrum radio quasars having significantly
steeper spectra than the BL Lac objects. The spectrum in both types of objects
flattens at the locations of the jet components indicating particle
acceleration or density enhancements along the jet. The mean spectral index at
the component locations of -0.81 corresponds to a power-law index of ~2.6 for
the electron energy distribution. We find a significant trend that jet
components with linear polarization parallel to the jet (magnetic field
perpendicular to the jet) have flatter spectra, as expected for transverse
shocks. Compared to quasars, BL Lacs have more jet components with
perpendicular magnetic field alignment, which may explain their generally
flatter spectra. The overall steepening of the spectra with distance can be
explained with radiative losses if the jets are collimating or with the
evolution of the high-energy cutoff in the electron spectrum if the jets are
conical. This interpretation is supported by a significant correlation with the
age of the component and the spectral index, with older components having
steeper spectra. (abridged)
