  Terahertz coloured digital holography ranging from 1.39 THz up to 4.25 THz is
demonstrated. It is shown that it can be applied for the investigation of
low-absorbing objects, and it is illustrated via inspection of stacked graphene
layers placed on high-resistivity silicon substrate. Holograms are recorded
using an optically-pumped molecular THz laser operating at discrete emission
lines of 1.39 THz, 2.52 THz, 3.11 THz, and 4.25 THz frequencies and it unveiled
that phase-shifting methods allow to qualitatively reconstruct coloured THz
holograms with improved quality achieved by removing unwanted information
related with so-called DC term and conjugated beam forming virtual image.
