is really more of a "there's something fishy about that floor tile" sense, and doesn't actually have anything to do with eyesight.
1. A small one, I guess. 2. Round. I think Diggles, really, are that Mustard/Yellow you generally see them in. Sure, there's colouration, but I think it's like hair colour for people - the majority of people have brown hair, but there are blondes, reds (<3), greys, whites and so on.
My feeling is that the iconic diggle is more of a bright, somewhat yellowish green than a light greenish yellow. Unfortunately, this color doesn't seem to have made it into the game yet. The poll choices seem to reflect the problem with diggles: yellow, beige, tan, taupe, mustard... the iconic color of a diggle is boring. (Like the nose?) That said, I like diggles lore-wise and shape-wise (no tuft, small rabbit tail), but color-wise, I only like the ones that aren't in the "iconic diggle" spectrum: the purple diggle is my favorite diggle that's in the poll, but I'd never call it the iconic diggle. However, I feel like a bright green diggle would both look good and be close enough to the iconic spectrum to act as the "normal" diggle. My second choice would be a French's Mustard yellow, I guess, it's in the iconic spectrum, and if a bit boring, at least it's not drab. That said, if you're making Merch, I think allowing us to taste the rainbow a bit would help sales greatly...you could even offer a discount "rainbow pack" with red, yellow, green, blue, and purple diggles to boost sales further.