The paper presents the results of measurements of the dielectric constant of composite materials, both solid and porous, containing multi-walled carbon nanotubes (MWCNT) as an inclusion, and water-dispersion color as a binder. The rectangular cavity was used to measurements. It is shown that the location of the sample in the cavity affects the accuracy of measurements. The permittivity spectra of the solid samples shows that the addition of nanotubes to the binder leads to shifting the dispersion range to the low-frequency region. The results show that materials can be used as radar absorbers.