One aspect of the green/yellow earth which seems to have changed over the years (at least in the U.K.) is the proportions of the colors. Look at the early use of this system here and the insulation is predominantly green with a yellow stripe, or at worst, 50/50. The proportions seem to have shifted so that now we have mostly yellow with just a thin green stripe. (I believe there is an IEC specification which says that one color must be no less than 30%, but this seems to be ignored frequently.)
The yellow/green sleeving used on fixed wiring here has the green stripe lengthways, so if the sleeve is turned at certain positions it can look as though it's plain yellow. It's only a minor point and it soon becomes obvious as to which wire is earth, but it's annoying to see this in a packed box where plain yellow is also used as a line color.
A similar change seems to have taken place with the neutral in appliance cords here. The original specification called for light blue, but the shade of blue used by many manufacturers seems to have become much darker in recent years.
C-H,
You mentioned the possible difficulty in disguishing between some colors, but I would also add that black and brown (particularly if dark brown) can sometimes be hard to tell apart as well. We already have 4-core flexible cords used for wiring heater controls and the like, and these are generally brown, black, blue, green/yellow. I've often been in dark corner and had to get some extra light to distinguish the two line colors.
