Head-on collision of droplets consisting of different liquids for a wide range of Weber numbers between 25 and 430 is studied numerically using the volume of fluid (VOF) methodology and by adopting an axisymmetric approximation. An adaptive local grid refinement technique is used to capture with precision the liquid air interface at significantly reduced computational cost relative to a uniformly dense grid. The conditions setting the boundary between permanent coalescence and reflexive separation are defined; in addition, useful relationships determining the ligament's elongation, the corresponding time of its breakup, the number of satellite droplets formed, the maximum droplet deformation, and the viscous dissipation coefficient are presented. Furthermore, the present study reveals the dependence of collision characteristics not only on Weber number, but also either on Reynolds, capillary, or Ohnesorge numbers.