Hi,
I've got an '83 Golf brochure and yellow isn't listed as a colour. Gti's were never offered in yellow and I think they stopped doing it on all Mk1's in about '77. I suppose it could be a special factory request - they did a couple of sand (gold) and atlas grey Mk1 Gti's as special orders, but these colours were available on non-Gti (sand) and Mk2's (grey). Never heard of a yellow one though. Take a door panel off and look at the inside of the door or lift the carpets and see if you spot another colour !
All the best,
Mark