This site explains the process of coloring comics during the 60's (when the Hulk started)
The possible combinations of these tints gave colorists a palette of 64 possible colors to use in the books, though most used no more than half of them. Many of the darker colors were indistinguishable in print.
This is the 64 color palette they had to work with during the 60's.
This is how they color them with color codes for printing.
This is the part is where I speculate. If I am limited by those colors and I have to color the Hulk and make him stand out among rocks, concrete buildings, roads, night scenes - I would ask for more grey color choices or just make him green :)
It would also be worth noting the silo color in the Superman strip (lower right hand panel) the dark grey is created by laying down 3 colors Yellow2 Red2 Blue3 (Y2R2B3).
Generally laying down one color is best, anything more than one color and registration problems, dot gain problems, paper quality problems, paper stretching problems, become more pronounced.
Also worth noting from http://www.comicartistsdirect.com/articles/coloring.html Comic companies on a tight budget were probably not willing to shell out the extra bucks for color proofs. The separators' work went right from rubylith overlays to the camera to shoot plates. That is why a plate is occassionally missing from a color in older comics.
Another article quotes http://goodcomics.comicbookresources.com/2006/03/02/comic-book-urban-legends-revealed-40/
A symptom of this cost-cutting is evident in the coloring process. The four-color separations and newsprint-esque pages in Marvel comics did not always bode well for certain color schemes.
One such color was grey.
It was not that comics could not use the color grey – they could. It simply wouldn’t come out the same way each time.
Therefore, when 1962’s Incredible Hulk #1 rolled around, Marvel had a problem. Dr. Bruce Banner is transformed into a monster called the Hulk by Gamma Radiation. The giant behemoth known as the Hulk is grey…but as you can see, the grey-coloring is far from consistent within the issue. Just compare this interior pages with the cover (thanks to H, from the Comic Treadmill, for the scans of the interior pages).
There are also current print problems with grey (kindly take note this is with current printing techniques)
1) Unlike instruments, we perceive color in the context of surrounding colors. If you look at the grey bar in this graphic:you'll see that it appears to change its tonality depending on whether it's against the red or green background even though the grey is in fact exactly the same across its width. You may also see a slight shift in color. This optical illusion is called "simultaneous contrast."
Even if the grey patch in a color bar appears perfectly neural compared with the 50%K patch - the grey within the live image area may not appear neutral due to the effect of surrounding colors.
2) Our perception of color, especially neutral grey, not only varies from individual to individual, but is also affected by age and gender. Older men (e.g. typical press operator) will not see a 3/C grey vs a 50% K grey the same as younger women (e.g. typical print buyer). You can test this yourself with a chart like this one:On the left is the reference 50%K. On the right are a variety of 3/C greys. Try and pick out which one is identical to the reference grey. In fact, if you were to print out the patches, cut them into squares, and asked co-workers to match the 3/C patch to the 50%K patch you'll most likely see the pattern of men/women, younger/older, choosing different 3/C patches that they see as perfectly matching the 50%K one.
Lack of instrument agreement
According to a report by Greg Radencic, PIA/GATF, on Spectrophotometer Device Agreement presented at a recent Splash technical color conference, out of seven spectrophotometers tested, four had a DeltaE error over 1.5 when measuring grey balance on 80lpb gloss coated paper. One instrument had a Delta E error of 1.75, another 3, another 4 and the last had a whopping Delta E error of 7 when measuring grey balance.
The seven instruments were factory new and calibrated.
The measured match assessment of 3/C grey and 50% K is inconsistent/unreliable between different types of spectrophotometers. Even different instruments of the same model may disagree.
The metameric issues
One final issue is that a chromatic 3/C grey (50C, 39M, 39Y) and an achromatic 50% black form a metameric pair. What this means is that they may match color under one lighting condition, however, under a different lighting the 3/C grey will shift hue because it is spectrally different than the 50% black. To illustrate:
Metameric issues can arbitrarily and continuously cause 3/C grey patches to mismatch against the 50%K reference patch as viewing conditions change.