Lizard

Nice picture, Geoff!
These lizards are capable of changing their body colors, similar like chameleons do. (actually many lizards have this capability, especially agamids). This is probably about as contrasty and colorful as this species gets. Often they turn to dark brown and blend in very well with the leaf litter.

The common names of the species in the Acanthosaura genus are a big mess. They are called mountain lizards, tree lizards, horned lizards, spiny lizards, pricklenape or a combination of these. Generally this particular species is called Cross-bearing Tree (mountain, spiny or whatever) Lizard.
The scientific name - Acanthosaura crucigera as Rushen mentioned - is therefore more useful. To my knowledge there is only one Acanthosaura species in Kaeng Krachan area. But there is a couple more species throughout the country, some quite recently split off from this A. crucigera. And there are still some waiting to be described.

Colors can change when in distress. And they might even inflate their throat a bit and open their mouth. It is quite amazing how quick they can change these colors.
Colors are likely also changing when they notice another lizard of same species. Either to attract or to warn this is their territory...
Sometimes with bobbing their heads.