  We select a sample of nearby Ultraluminous X-ray sources with long XMM-Newton
observations and analyse all the available XMM-Newton data using both X-ray
spectral fitting techniques and hardness-intensity diagrams. The sample
includes IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg II
X-1, NGC 1313 X-1, NGC 1313 X-2 and NGC 253 X-1. We found that, although a
common reference model can be used to describe the X-ray spectra, the sources
show different spectral evolutions, phenomenologically described in terms of
variations in the properties of a soft component and a high energy tail.
Variations at low energies are accounted for (mostly) by changes in the
normalization of the soft component and/or in the column density to the source,
while variations in the high energy tail by changes in the parameters of an
optically thick corona. This spectral variability is rather well characterized
on a colour-colour and hardness-intensity diagram in terms of suitably defined
hardness ratios. We suggest the existence of a variability pattern on the
hardness-intensity diagram and we interpret it in terms of the switch between a
near-Eddington and a super-Eddington accretion regime. The transition between
the two regimes seems to be driven mostly by changes in the contribution of the
soft component, which can be explained in terms of the increasing importance of
wind emission. The analysis is complemented by an investigation of the
short-term time variability of all the sources. In general, no clear
correlation between the spectral and temporal properties is found.
