  In this letter we address the problem of the origin of blue cores and inverse
color gradients in early-type galaxies reported in the Hubble Deep Field North
and South (HDFs) by Menanteau, Abraham & Ellis 2001. We use a multi-zone single
collapse model. This model accounts for the observed blue cores by adopting a
broad spread in formation redshifts for ellipticals, allowing some of these
galaxies begin forming no more than ~1 Gyr before the redshift of observation.
The single-zone collapse model then produces cores that are bluer than the
outer regions because of the increase of the local potential well toward the
center which makes star-formation more extended in the central region of the
galaxy. We compare the predicted V-I(r) color gradients with the observed ones
using the redshift of formation ($z_F$) of the elliptical as the only free
parameter. We find that the model can account with relatively good agreement
for the blue cores and inverse color gradients found in many spheroidals and at
the same time for the red and smooth colors profiles reported. Based on the
model our analysis suggests two populations of field ellipticals, one formed
recently, within $\lesssim1$Gyr and another much older formed $\gtrsim4$Gyr
since the redshift of observation.
