There are two variants of Fantastic Four #110 shown below, the "green" error variant and the corrected variant:
There are many comics with printing variances, where a certain color runs out, however, in this case we have a printing error where two of the 4 ink colors (magenta, cyan, yellow and black)
used to create all the colors on the cover (they are mixed at various percentages) were applied to the wrong plates before printing.
The "green" effect comes from mixing cyan and yellow instead of magenta and yellow; switching the inks also gave the FF an interesting costume color!
Despite the age of these comics, the error variant is still reasonably easy to get hold of with a small number available on various auction sites at any one time.
At the time of writing (updated in August 2015) there were 56 error variants graded by CGC with three copies at the top in 9.6 NM+ (Near Mint Plus). Interestingly these are registered as a variant
in the CGC census whereas a similar error on Avengers #10 is recorded as qualified in the census (maybe CGC will correct this inconsistency in time).