  We analyse colours of the nuclear regions of intermediate redshift disk
galaxies, with the aim of obtaining empirical information of relative ages of
bulges and disks at 0.1 < z < 1.3. We work with an apparent-diameter limited
parent sample of 248 galaxies from the HST Groth Strip Survey. We apply a
conservative criterion to identify bulges and potential precursors of
present-day bulges based on nuclear surface brightness excess above the
exponential profile of the outer parts. We measure bulge colours on wedge
profiles opening on the semi-minor axis, and compare them to disk, and global
galaxy colours. For 60% of galaxies with bulges, the rest-frame nuclear colour
distribution shows a red sequence that is well fit by passive evolution models
of various ages, while the remainder 40% scatters towards bluer colours. In
contrast, galaxies without central brightness excess show typical colours of
star forming population and lack a red sequence. We also see that, as in the
local Universe, most of the minor axis colour profiles are negative (bluer
outward), and fairly gentle, indicating that nuclear colours are not distinctly
different from disk colours. This is corroborated when comparing nuclear,
global and disk colours: these show strong correlations, for any value of the
central brightness prominence of the bulge. Comparison with synthetic models of
red sequence bulge colours suggests that such red bulges have stopped forming
stars at an epoch earlier than ~ 1 Gyr before the observation. The correlation
between nuclear and disk colours and the small colour gradients hints at an
intertwined star formation history for bulges and disks: probably, most of our
red bulges formed in a process in which truncation of star formation in the
bulge did not destroy the disk.
