So you want something like this, I take it?
Since the colours are mostly defined by the reflection, your requirement of the light source being above the beetle doesn't say much about how the colours on the beetle are perceived, so I'll assume it is also observed from above.
The typical colours are caused by iridescence, which makes it so colours change depending on the viewing angle and angle of incidence of light.
Note that what colours you see are those of the sky reflected (in slightly altered hues)—the dark parts don't reflect enough light to change the colour, and you can see a reflection of what are possibly trees on the sides of the dung beetle. If the carapace will remain in a single area, take this into account, and imagine how the reflections change based on that area (e.g. in front of a window looking outside, the blue hue would be visible towards the lit side of the carapace).
What will work for a static effect is to paint the carapace black, then spray-paint a (greenish) blue gradient towards the centre or zenith of the carapace. Smoother areas can be spray-painted further away from the surface, and irregular surfaces might require more precise and closer spraying, and possibly manual brush work.
You can imitate a striated structure like the one seen in the image above by leaving a few stripes black. What will make the effect pop is to have a more saturated blue colour towards one side of the shield, and a saturated green towards another (you can see this most clearly along the 'collar' of the beetle, and other detailed areas).
Afterwards, I'd still recommend using a gloss varnish to cover it all.
If the varnish is really glossy, you might even consider painting the carapace black and only applying a glossy varnish afterwards, so the reflections will actually change in real-time.