Context : Spectral variability is the main tool for constraining emission models of BL Lac objects .

Aims : By means of systematic observations of the BL Lac prototype PKS 2155-304 in the infrared-optical band , we explore variability on the scales of months , days and hours .

Methods : We made our observations with the robotic 60 cm telescope REM located at La Silla , Chile .
VRIJHK filters were used .

Results : PKS 2155-304 was observed from May to December 2005 .
The wavelength interval explored , the total number of photometric points and the short integration time render our photometry substantially superior to previous ones for this source .
On the basis of the intensity and colour we distinguish three different states of the source , each of duration of months , which include all those described in the literature .
In particular , we report the highest state ever detected in the H band .
The source varied by a factor of 4 in this band , much more than in the V band ( a factor \approx 2 ) .
The source softened with increasing intensity , contrary to the general pattern observed in the UV-X-ray bands .
On five nights of November we had nearly continuous monitoring for 2-3 hours .
A variability episode with a time scale of \tau \approx 24 h is well documented , a much more rapid flare with \tau =1-2 h , is also apparent , but is supported by relatively few points .

Conclusions : The overall spectral energy distribution of PKS 2155-304 is commonly described by a synchrotron-self-Compton model .
The optical infrared emission is however in excess of the expectation of the model , in its original formulation .
This can be explained by a variation of the frequency of the synchrotron peak , which is not unprecedented in BL Lacs .