We observed the field of GRB 100728A (Swift trigger 430151; Cannizzo et al., GCN 11004; A. von Kienlin, GCN 11006) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile). Observations started at 09:26 UT, 7 hours after the GRB trigger and were terminated at 10:32 UT by the GRB 100728B trigger. They were performed at an average seeing of 1.3" and at an average airmass of 2.5. We found a single, very faint point source in the H band within the enhanced Swift-XRT error circle reported by Beardmore et al. (GCN 11005) at RA (J2000.0) = 05:55:02.01 DEC (J2000.0) = -15:15:20.4 with an uncertainty of 0.5" in each coordinate. Based on 24 min of total exposures in g'r'i'z' JHK, we estimate preliminary magnitudes and upper limits (all in AB system) of g' > 23.3 r' > 23.9 i' > 23.5 z' > 23.5 J > 22.0 H = 21.2 +- 0.3 K > 20.3 Given magnitudes and upper limits are obtained by calibrating the images against GROND zeropoints as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.18 mag in the direction of the burst (Schlegel et al. 1998). These magnitudes imply very red colors of the afterglow of J-H > 0.8 and r-H > 2.7. Together with the significant excess absorption seen by the XRT (Evans & Cannizzo, GCN 11014), and the lack of prompt detections in the optical and UV filters (Oates & Cannizzo, GCN 11016), these values are suggestive of a highly extinguished optical/NIR afterglow. The K band upper limit is not constraining even for the steepest SED slopes and consistent with this hypothesis.