The rest-frame UV-optical (i.e., NUV - B) color is sensitive to both low-level recent star formation (specific star formation rate-sSFR) and dust. In this Letter, we extend our previous work on the origins of NUV - B color gradients in star-forming galaxies (SFGs) at z similar to 1 to those at z similar to 2. We use a sample of 1335 large (semimajor axis radius R-SMA > 0.'' 18) SFGs with extended UV emission out to 2R(SMA) in the mass range M-* = 10(9)-10(11) M-circle dot at 1.5 < z < 2.8 in the CANDELS/GOODS-S and UDS fields. We show that these SFGs generally have negative NUV - B color gradients (redder centers), and their color gradients strongly increase with galaxy mass. We also show that the global rest-frame FUV - NUV color is approximately linear with A(V), which is derived by modeling the observed integrated FUV to NIR spectral energy distributions of the galaxies. Applying this integrated calibration to our spatially resolved data, we find a negative dust gradient (more dust extinguished in the centers), which steadily becomes steeper with galaxy mass. We further find that the NUV - B color gradients become nearly zero after correcting for dust gradients regardless of galaxy mass. This indicates that the sSFR gradients are negligible and dust reddening is likely the principal cause of negative UV-optical color gradients in these SFGs. Our findings support that the buildup of the stellar mass in SFGs at Cosmic Noon is self-similar inside 2R(SMA).