It's not a paper variety, it's what it should be. Gibbons #139, Michel #77 & Scott #78 all agree, printed on green paper. None of these catalogues, particularly my Michel Specialised, mention a white/cream paper.
However, a quick perusal of Delcampe shows most copies on green paper, but a few are on white/cream & it doesn't look like problems with scanning, & if the paper colour was water-soluble you'd think there would be at least some streaks on used copies.
Here's some examples from Delcampe showing paper variations

Here's an example from Delcampe with the catalogued green paper
