Trigun Blue eyes (Spoiler)

Sorry, I'm just curious(and my memory isn't that good). Why do Vash's eyes glow blue in the Diablo episode?
Is it because he is a plant?

If someone could tell me, I've watched the whole series but some things still puzzle me.:dizzy2:

 

I thin kthat it may be because he is a plant, which I think is kind of weird >.<x But as well he was really really ticked off and sometimes my eyes glow too when I'm ticked! (I wish...)