My tuppence worth,
i have seen this colour scheme on Muirwoods from 1989-1991 with subtle variations. There are Muirwoods with all neon stems bars and forks, there are ones with black stems and bars the rest yellow, there are also ones with black stems and yellow bars and forks. There are Muirwoods with silver decals, there are also some with the american flag and some without. I also think there is some variation in the yellow neon colour but cant be sure this isnt down to fading. There is also some variation in the font used and the thickness of the lettering. There are also 1990 Muirwoods in splatter paint type finish which i think were made for the american or european market.
My bike actually had black spinner forks and bars and a yellow stem. My bike i got second hand in 1990. It had been well used and abused at that time, it also has the rear mounted ubrake. this was allegedly used for heel clearance on the smaller frames, but stopped after 1989. but then i have seen size 16 neon yellow muirwoods with canti's on the rear. Mine did come with these shifters though.
the Muirwoods for 1991 had moved to the zolatone paint scheme.
So what do i think? I think there are Neon yellow Muirwoods from 1989-1990 ish that were still being sold in shops up to 1991. It was the entry level bike and perhaps proved very popular so maybe got a slightlylonger run in this popular neon colour scheme, but then im guessing and dont really know.
My serial number did not make sense ,but Debbie from the UK Marin distributor said they did not run to the format outlined about until early in the 90's