  The rest-frame UV-optical (i.e., NUV-B) color index is sensitive to the
low-level recent star formation and dust extinction, but it is insensitive to
the metallicity. In this Letter, we have measured the rest-frame NUV-B color
gradients in ~1400 large ($\rm r_e>0.18^{\prime\prime}$), nearly face-on
(b/a>0.5) main-sequence star-forming galaxies (SFGs) between redshift 0.5 and
1.5 in the CANDELS/GOODS-S and UDS fields. With this sample, we study the
origin of UV-optical color gradients in the SFGs at z~1 and discuss their link
with the buildup of stellar mass. We find that the more massive, centrally
compact, and more dust extinguished SFGs tend to have statistically more
negative raw color gradients (redder centers) than the less massive, centrally
diffuse, and less dusty SFGs. After correcting for dust reddening based on
optical-SED fitting, the color gradients in the low-mass ($M_{\ast}
<10^{10}M_{\odot}$) SFGs generally become quite flat, while most of the
high-mass ($M_{\ast} > 10^{10.5}M_{\odot}$) SFGs still retain shallow negative
color gradients. These findings imply that dust reddening is likely the
principal cause of negative color gradients in the low-mass SFGs, while both
increased central dust reddening and buildup of compact old bulges are likely
the origins of negative color gradients in the high-mass SFGs. These findings
also imply that at these redshifts the low-mass SFGs buildup their stellar
masses in a self-similar way, while the high-mass SFGs grow inside out.
