  We present calculations for attosecond atomic delays in photoionization of
noble gas atoms based on full two-color two-photon Random-Phase Approximation
with Exchange in both length and velocity gauge. Gauge invariant atomic delays
are demonstrated for the complete set of diagrams. The results are used to
investigate the validity of the common assumption that the measured atomic
delays can be interpreted as a one-photon Wigner delay and a universal
continuum--continuum contribution that depends only on the kinetic energy of
the photoelectron, the laser frequency and the charge of the remaining ion, but
not on the specific atom or the orbital from which the electron is ionized.
Here we find that although effects beyond the universal IR--photoelectron
continuum--continuum transitions are rare, they do occur in special cases such
as around the $3s$ Cooper minimum in argon. We conclude also that in general
the convergence in terms of many-body diagrams is considerably faster in length
gauge than in velocity gauge.
