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The Planck-ATCA Co-eval Observations (PACO) have provided multi-frequency(5-40 GHz) flux density measurements of complete samples of Australia Telescope20 GHz (AT20G) radio sources at frequencies below and overlapping with Planckfrequency bands, almost simultaneously with Planck observations. In this workwe analyse the data in total intensity for the spectrally-selected PACO sample,a complete sample of 69 sources brighter than 200 mJy at 20 GHz selected fromthe AT20G survey catalogue to be inverted or upturning between 5 and 20 GHz. Westudy the spectral behaviour and variability of the sample. We use thevariability between AT20G (2004-2007) and PACO (2009-2010) epochs todiscriminate between candidate High Frequency Peakers (HFPs) and candidateblazars. The HFPs picked up by our selection criteria have spectral peaks > 10GHz in the observer frame and turn out to be rare (<0.5% of the S >200 mJysources), consistent with the short duration of this phase implied by the`youth' scenario. Most (~ 89 %) of blazar candidates have remarkably smoothspectra, well described by a double power-law, suggesting that the emission inthe PACO frequency range is dominated by a single emitting region. Sources withpeaked PACO spectra show a decrease of the peak frequency with time at a meanrate of -3 +- 2 GHz/yr on an average timescale of \tau = 2.1 +- 0.5 yr. The5-20 GHz spectral indices show a systematic decrease from AT20G to PACOobservations. At higher frequencies spectral indices steepen: the median \alpha30-40 GHz is steeper than the median \alpha 5-20 GHz by \delta\alpha = 0.6.Taking further into account the WISE data we find that the SEDs, \nu S(\nu), ofmost of our blazars peak at \nu_p <10^5 GHz; the median peak wavelength is\lambda_p ~93 \mu m. Only 6 have \nu_p>10^5 GHz.

The Planck-ATCA Co-eval Observations (PACO) project: the spectrally-selected sample