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We select a sample of nearby Ultraluminous X-ray sources with long XMM-Newtonobservations and analyse all the available XMM-Newton data using both X-rayspectral fitting techniques and hardness-intensity diagrams. The sampleincludes IC 342 X-1, NGC 5204 X-1, NGC 5408 X-1, Holmberg IX X-1, Holmberg IIX-1, NGC 1313 X-1, NGC 1313 X-2 and NGC 253 X-1. We found that, although acommon reference model can be used to describe the X-ray spectra, the sourcesshow different spectral evolutions, phenomenologically described in terms ofvariations in the properties of a soft component and a high energy tail.Variations at low energies are accounted for (mostly) by changes in thenormalization 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 anoptically thick corona. This spectral variability is rather well characterizedon a colour-colour and hardness-intensity diagram in terms of suitably definedhardness ratios. We suggest the existence of a variability pattern on thehardness-intensity diagram and we interpret it in terms of the switch between anear-Eddington and a super-Eddington accretion regime. The transition betweenthe two regimes seems to be driven mostly by changes in the contribution of thesoft component, which can be explained in terms of the increasing importance ofwind emission. The analysis is complemented by an investigation of theshort-term time variability of all the sources. In general, no clearcorrelation between the spectral and temporal properties is found.

Ultra Luminous X-ray Sources: a deeper insight into their spectral evolution