  The study of the visible colours of the trans-Neptunian objects opened a
discussion almost 20 years ago which, in spite of the increase in the amount of
available data, seems far from subside. Visible colours impose constraints to
the current theories of the early dynamical evolution of the Solar System such
as the environment of formation, initial surface composition, and how (if) they
were scattered to regions closer to the inner planets. In this paper we present
an updated version of our database of absolute colours and relative phase
coefficients for 117 objects. We define the absolute colours as the difference
of the absolute magnitudes $H_V-H_R$, and the relative phase coefficient as the
difference of the slopes of the phase curves $\Delta\beta$. These were obtained
joining our own observations plus data from the literature. The methodology has
been introduced in previous works and here we expand in some interesting
results, in particular the strong anti-correlation found between $H_V-H_R$ and
$\Delta\beta$, which means that redder objects have steeper phase curves in the
R filter, while bluer objects have steeper phase curves in the V filter. We
analyse a series of results published in the literature in view of our
database, which is free of phase effects, and show that their statistical
meaning is not very strong. We point out that phase-colouring and observational
errors play an important role in the understanding of these proposed
relationships.
