Cheers Malcolm! I'll admit that if I wanted a knock-out view of a comet however bright (but not too big!) I'd choose a 20" SDM over a 4.5" Tasco any day! However that's a pretty mono-dimensional view of astronomy and if I wanted to do accurate, formal comet observations I'd use the appropriate instrument, in this case binoculars, and naked-eye if it gets much brighter. The problem with big aperture on bright comets is that it distorts the relative magnitude in something called the "aperture effect". This can be corrected but probably not in such a mismatch as 20" versus binoculars! I'd also be extremely surprised if you're not actually seeing a vast coma through the 20", after all light is light! The problem may be that the gradient is imperceptible and there is no clear end to it - but there is a noticeable gradient change at similar diameter to what you reported. That should be clear to you and probably has the appearance of the limits of the coma.
As far as the colour goes, the light-adapted human eye is optimised to 555nm (in the green) and this may assist in picking up greenish colour in bright comets in relatively small apertures.
Cheers -