The first-level illusionist spell color spray operates by overloading the senses/neural networks of its subject. Our view is that the same lack of conventional consciousness that makes the undead immune to sleep and charm also makes them immune to color spray. Would love to hear different interpretations, or assent.
This is a by-the-book First Edition AD&D game (PHB, DMG, no orange spines allowed).
First and foremost, consider the nature of the works you cite. When 1st edition Advanced D&D (aka 1e) was written, roleplaying was still new and error-correction and (extensive) rules extrapolations had not yet occurred. Worse yet, this rules set was written while Original D&D was being played by everyone, including the 1e author Gary Gygax. Several errors (see footnote) occurred during this overlap phase, and a general lack of specificity is common, most rules being left to the DM's logic and discretion.
Any question demanding a "by-the-book" (BtB) 1e ruling is thus subject to these errors and omissions, and the answer may be contradictory and/or illogical.
The first-level illusionist spell color spray operates by overloading the senses/neural networks of its subject.
Sorry Rich, but 'overloading the senses' is your deduction, and is not BtB. We can only say (BtB) that the 1st level spell (i.e. minimum power) causes unconsciousness, blindness, or stun, and that some victims get no saving throw.
Our view is that the same lack of conventional consciousness that makes the undead immune to sleep and charm also makes them immune to color spray.
While this is a logical sentiment it again is not supported by the Rules As Written. Nowhere do the rules refer to a 'lack of conventional consciousness'. Although it is a logical deduction, your BtB emphasis again precludes it.
It is true that most undead are immune to sleep and charm effects. But note that a Ghost is not explicitly given those immunities. This was probably a mere omission -- but again we have the BtB issue. (Deductions about immunities of undead as a class may also be faulty; many undead are also immune to hold and cold effects, but the Ghost is not, nor are ghouls or ghasts.)
THUS: given the descriptions in Monster Manual (1977) and lacking any explicit prohibition in the subsequent volumes, the Players Handbook (1978) and Dungeon Master's Guide (1979), the BtB ruling should be that Color Spray affects all undead mentioned in those works (Skeleton, Ghoul, Shadow, Wight, Ghast, Wraith, Mummy, Spectre, Vampire, Ghost, Lich).
In my own games I apply additional deductions and agree with your stance. But strictly BtB I must respectfully disagree with the other answers offered here.
Corollary: Don't insist on BtB answers for 1e. ;)
As an AD&D DM - I might rule differently. But, your dungeon master has house-ruled this result, so that's the call. I'd support him/her in this call and suggest out of game discussion amongst the group if dissent remains.
You didn't present your interpretation. Preparing an alternate explanation might help.
If you're ever a DM, I hope your players will support your decisions. For myself (and myself only) I usually say "yes" - I now tend to rule in favor of the PC that everything "works as described" in all cases, unless there is explicit disqualifying clause. But, I didn't always DM this way - there was a day when I would have ruled exactly as your DM did.
It was over time that I learned to be a better, more interactive DM that I learned to interpret the rules for fun over a logical/strict rules mindset. Most DMs go through this same learning cycle.
It's how we played when we played first edition. If I remember correctly, we had some limitations - higher level illusion spells were more effective on lower level undead, and more complete illusion spells had a higher chance of success, but, generally, when we ran into undead creatures, we stayed away from illusions.
