Normally with a metal figure I'd use a good primer. Traditionally many painters use a black primer for dark figures and a white primer for light or vibrantly coloured figures. I've even seen a fascinating technique where figures are sprayed with black primer from below, and white primer from above to give a built in shadow effect.
Reaper Bones are supposedly made from a similar plastic to acrylic paint. This means they shouldn't need a primer. But most plastics are hydrophobic, this means that water doesn't adhere well to them, instead it beads and tries to separate from the surface. Bones are no different, if you water the base coat down, paint doesn't stick to the figures very well. If you use a paint that's too thick, it risks obscuring detail on the figure, so it's tempting to use a flow medium, or water the paint down in some way...but even using a flow medium to thin the paint causes it to repel from the plastic of the figures (much like it would do from a poorly primed metal or resin figure).
I decided to go with a pigment heavy paint, something that had very little medium, and gave good coverage with a thin layer (discontinued stock from Rackham paints). After finding the paint still not completely sticking to the surface, I thinned it in the ratio of 2 parts paint : 1 part flow medium : 1 part PVA glue.
Since the majority of the figures will be white, I'm undercoating them with the darkest shade of the armour...the colour that the armour's shadows would be. If these were dirty mercenaries, I'd go with a murky grey or brown, but since these are intended to look like clean, pristine, and cold peacekeepers/police, I'm going with a sky blue (more specifically "Wizard Blue" from the Rackham Range). Areas of the figures that will be metallic are left unpainted at this stage. We'll get to them next.