Another is it extraordinary light blue or greenish colour against the starry field. Its colour reminds me of either NGC 3195 in Chamaeleon or NGC 3132 in Vela, though neither of these is not as prominent as NGC3918. I have on many occasions used it for a typical example of a planetary nebula for my Astronomy classes and during observation sessions. At first glance using low power the tiny 'even-light' circular disk reveals no structural details. Uranus - however it is not as green.
Larger apertures reveals the very blue colour, but no structure - even using an O III filter.
It is classed as a type 2b in the 1934 Vorontov and Velyaminov classification indicating a uniform distribution of brightness. A small knot or bulge, about PA 250 O is suspected towards the edge of the planetary, but I have never seen it. For apertures above 40cm. Because of the planetary's overall brightness the PNN is invisible to telescopes below about 60cm. Yet other catalogues state that the central star is more like 15.6.