Context : Spectral index studies of halos , relics , and radio galaxies provide useful information on their origin and connection with merger processes .

Aims : We present WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm , 85 cm , and 2 m. The spectral index images allowed us to study the integrated spectrum of halo and relic and to investigate the physical properties of the Beaver head-tail radio galaxy belonging to the cluster .

Methods : In the radio halo , the spectral index is steeper at the center and flatter at the locations of the radio filaments , clearly detected at 25 cm .
In the relics , the spectral index flattens , moving away from the cluster center .
For the Beaver radio galaxy , the spectrum severely steepens from the head towards the end of the tail , because of the energy losses suffered by the relativistic particles .

In the 2 m map , which is the first high-sensitivity image presented in the literature at such a long wavelength , a new Mpc-size emission region is detected between the known radio halo and the NW relic .
Not detecting this feature in the more sensitive 85 cm observations implies that it must have a very steep spectrum ( \alpha \leq - 2.6 ) .

Results : The observational properties of the radio halo suggest that either we are looking at a superposition of different structures ( filaments in the foreground plus real halo in the background ) seen in projection across the cluster center or that the halo is intrinsically peculiar .
The newly detected extended region to the NW of the halo could be considered as an asymmetric extension of the halo itself .
However , since radio halos are known in the literature as structures showing a regular morphology , the new feature could represent the first example of steep Mpc-size diffuse structures ( MDS ) , detected around clusters at very low frequencies .

Conclusions :