I wouldn't say oak green golfs are really worth more but due to the fact that some people view a late oak green example as the epitomy of the mk2 this has pushed prices up. As long as they remain desirable by enthusiasts then the prices will remain higher than other colour cars. There are definately much rarer colours out there but if there's no demand for them the price will reflect that. I've owned a couple of oak green golfs and think the colour suits the mk2 down to the ground.
What you are prepared to pay is really down to the individual.
Both of mine have been 3door 16V's, for my last one on a h plate with 108k on the clock I paid £3050, for my current one on a j plate with 94k on the clock I paid £3295.
I think the colour definately looks nice but it needs the right conditions.
Being a dark colour it shows a layer of dust up really easy.
Oak green also needs a lot of light to bring it to life, on dull overcast days it just looks quite black. However on a nice bright sunny day the green really comes out and the colour looks fantastic!!!
These are just my views.
Toby