  Collisional ring galaxies are the outcome of nearly axisymmetric high-speed
encounters between a disc and an intruder galaxy. We investigate the properties
of collisional ring galaxies as a function of the impact parameter, the initial
relative velocity and the inclination angle. We employ new adaptive mesh
refinement simulations to trace the evolution with time of both stars and gas,
taking into account star formation and supernova feedback. Axisymmetric
encounters produce circular primary rings followed by smaller secondary rings,
while off-centre interactions produce asymmetric rings with displaced nuclei.
We propose an analytical treatment of the disc warping induced by an
inclination angle greater then zero. The star formation history of our models
is mainly influenced by the impact parameter: axisymmetric collisions induce
impulsive short-lived starburst episodes, whereas off-centre encounters produce
long-lived star formation. We compute synthetic colour maps of our models and
we find that rings have a B-V colour typically ~0.2 mag bluer than the inner
and outer disc, in agreement with observations.
