See Corrupt Octopus.

This is an attempt at a more straightforward corrupt design at common.

Oh! That's an interesting mechanic.

I think you'd want corruption counters to implement it; but that's probably fine.

I'm not opposed using +1/+1 counters.

Corrupt is very much a black concept. The ability could be any color, but the name isn't right

I'm not going for corrupt as in evil, but instead trying to suggest being mono-color is the creature's pure state. The creature taking on a second color, become multicolor, is a corruption to the creature's initial state.

That's not the connotation of that word in English, though. Corruption will always be read as evil or malicious

Even "taint" has negative associations. Maybe "influence"? "tint"? "distort"? In a world like Eldritch Moon where there's something warping all colours, it could work.

Tint works.

Perhaps citing a definition from Merriam-Webster will defend my stance: "to alter from the original or correct form or version."

From this I learned of "vitiate," a word I was unfamiliar with. That word would suffice.

Perhaps you should also look up the definitions of denotation and connotation while your over there. If aomeone tells you the issue is connotation and another mentions negative associations, then citing a definition is kinda missing the point.

Citing a (single) dictionary definition (out of many) is also kinda iffy. I usually use another dictionary, so let me try. I'll use the first definition from your source (Miriam-Webster) for both the intransitive and transitive version rather than cherry-pick one definition; usually definitions are sorted so the lower numbers are more commonly used forms - which are hence more relevant for connotation (which influences how the keyword will feel to your players).

transitive 1.a: > to change from good to bad in morals, manners, or actions

intransitive 1.a: > to become tainted or rotten

I will also point out that the definition you provide comes with an example: "The file was corrupted." What connotations does this example evoke?

If you want a value-neutral term here you don't have to look any farther than the definition you have brought up: alter.

Now I personally don't think corruption necessarily means from "towards evil", but certainly implies "towards bad" in the sense of "towards a lessened state", "losing (its original) value".

"Change" is a scary thing, because it is not always good and not always in our control, actually more often than not in someone elses control (which is why corrupt is (also) a transitive verb). And while change also can be for the better, we tend to call change that we disapprove of "corruption".

Now there is nothing wrong with using corrupt, but then I would make sure the thematic treatment acknowledges the connotations.

One could set up a corrupt vs. pure theme (listed as antonym to the adjective definition on Miiam-Webster). What would corrupt be? Multicolored? Having a mismatch between your color(s) and your 'base color(s)'?

I think you are throwing away a lot of potential when you go with definition 4, but ignoring the part that says "to alter from [...] correct form or version"