Found this species today, which appears to habe blueish-green scaling on the thorax. I can only imagine this as Matapa sasivarna, though there is a second species with blue/green markings and that is Zela smaragdina. The markings are considerably smaller than those on the only other specimen of M. sasivarna I have seen personally, and that was in Ranong.
I am pretty sure it is the first named, but not positive, though the pale apical forewing areas (definitely there, not a trick of the light) worry me a bit! Whichever, it would be a +1 for Samui.