I seem to remember being told at the time that this was M. madagascariensis, but M. baroni also occurs in the area. I didn't know you needed a ventral view to get a 100% ID. My impression is that this is better for M. baroni, though.
Mantella is a genus of frogs in the family Mantellidae, endemic to Madagascar. These are small frogs typically reaching 2–3 centimetres (0.79–1.18 in), with iridescent colors with combinations of black, blue, orange, yellow, and green which reflect the toxic nature of their skins. Several species in the genus are threatened because of habitat loss and over-collection for the international pet trade.