The red-and-white stripes separating the sides from the bottom visible in the first snake seem to generally be considered conclusive evidence for T. fucatus, and perhaps evidence that it's a male (though maybe females have these stripes in some populations, according to some authorities).
The sharp delineation between the reddish top-of-tail and green bottom-of-tail in the second snake are often cited as evidence in favor of T. nebularis. But it seems quite surprising that the two nearly-identical species would be found within ten feet of the other (you'd expect selection pressures to have favored one or the other). And also, the second snake's eyes are definitely more yellow than green, which seems to rule out T. nebularis. So I'm just going to call them both T. fucatus.
But also, some references claim that T. fucatus has more of a matte finish when photographed with a flash, whereas T. nebularis has more of a glossy finish. This snake looks pretty glossy to me (based on the extra-shiny flash highlights). On the other hand, it was a very wet night so maybe the snake is extra reflective just because it's wet. I'm soooo confused!
- Trimeresurus fucatus account on The Reptile Database
- Popeia fucata account on Amphibians & Reptiles of Peninsular Malaysia
- Popeia fucata account on Ecology Asia
- Das, I. 2012. A Naturalist's Guide to the Snakes of South-east Asia
- Das, I. 2010. A Field Guide to the Reptiles of South-east Asia