  Context: We use phase curves of small bodies to measure absolute magnitudes
and, together with complementary theoretical and laboratory results, to
understand their surfaces' micro and macroscopic properties. Although we can
observe asteroids up to phase angles of about 30 deg, the range of phase angles
covered by outer solar system objects usually does not go further than 7 to 10
deg for centaurs and 2 deg for trans-Neptunian objects, and a linear relation
between magnitude and phase angle may be assumed.
  Aims: We aim at directly comparing data taken for objects in the inner solar
system (inside the orbit of Jupiter) with data of centaurs and trans-Neptunian
objects.
  Methods: We use the SLOAN Moving Objects Catalog data to construct phase
curves restricted to phase angles less than or equal to 7.5 deg, compatible
with the angles observed for the trans-Neptunian/Centaur population. We assume
a linear model for the photometric behavior to obtain absolute magnitudes and
phase coefficients in the ugirz, V, and R filters.
  Results: We obtained absolute magnitudes in seven filters for $>4000$
objects. Our comparison with outer solar system objects points to a common
property of the surfaces: intrinsically redder objects become blue with
increasing phase angle, while the opposite happens for intrinsically bluer
objects.
