So there are several ways one could interpret removing a colour.

First of all, we need to establish what is meant by the term "colour".

The answer that comes to mind first, is also one of the hardest to rigidly define, so I'll talk about it last.

Firstly, you could be referring to the colour cones within the retina: Red, Green, and Blue. If this is how we define a colour, then removing one of them is really simple to think about, because it's a real medical condition that exists, 'Dichromacy' Is a medium-severity form of CVD (Colour vision deficiency) that occurs when someone is missing one type of colour cone (Or, more likely, it is not functioning). If this is the chosen definition. then I would give everybody in the word 'Tritanopia', where the blue colour cone is deficient. This also happens to be the rarest form of dichromacy, but I chose it because it maintains a lot of the lighter colours, and results in a lot of colours resembling what, in normal vision, are light pinks and whites, and ironically, blues, whereas the other two forms of dichromacy (i.e. where the red cone does not work - 'Protanopia, or the green cone does not work - 'Deuteranopia')

The next easiest definition be the removal of a continuous range of wavelengths from the world.

Okay, removing entire categories of photons is probably going to have some side effects, so let's say that the lenses in the human eye will filter out these wavelengths specifically. For a lot of the range, this actually means the colour isn't lost forever. We can replicate it the same way monitors trick out eyes, by combining other wavelengths to trigger the colour cones in our eyes the right amount, however, sufficiently sized ranges, and in particular areas, would make it much harder to replicate the colour using only wavelengths that are now valid, so some true colour loss may be achieved.

By this definition I would remove the lowest ends of the visible light spectrum. We sacrifice a lot of the blues and purples, which is sad, but ultraviolet light is not good for the eyes, and it would be a decent enough health benefit to the world to be worth it.

Next, we could supposedly just ban certain hex codes from monitors. The colours wouldn't be gone from the real world, but they'd be gone from the internet and that's all that matters really right? Firstly, as you said removing one colour, I'll restrict what we can ban to being a range of hue angles in the HSL colour space. In that case, I'd remove the weird 'green-ish-yellow-but-not-quite-lime' region at around 80-85°. All uses of it would look better with real lime, sorry.

Lastly, we tackle what people actually mean by 'colour'. I.e. Red, Yellow, Orange, etc. Richard of York's own personal encyclopaedia.

The boundaries between these colours are completely poorly defined. It all happens in the vision processing part of our brain and brains are complicated. The same colours can look different under different lighting and in different contexts, and on top of that language is complicated. There are general colours, colours that exist only in some languages, some that only exist in others, some colours that aren't even hues (black, gray, white, etc), some colours that aren't even colours but textures instead (silver). and if I really wanted to, I could add so many adjectives to qualify the colour that it's too specific to cause any meaningful change.

Because of this, I'm going to have to come up with some arbitrary decisions. The list of colours that exist will be, as referenced earlier, those of Richard of York: Red Orange Yellow, Green, Blue, Indigo, Violet. Except I'm going to collapse indigo and violet into a single Purple.

Now there are certainly some inconsistencies in this. But I think if you got a bunch of things, asked people to point at them and name their colour in a single word, they're much more likely to use these words than "Lime", "Cyan", or "Violet". I'll also add White, Grey, and Black to cover the 'pseudo-colours'.

That test for what's worth putting in the list also helps with our second problem. How do we map perceptions to these discrete colour words?
As we're already hacking into the brain, we'll use some smart magic, that detects when somebody would otherwise think "That's <colour>". As colour is a subjective experience, despite the amount of hand-wavyness, I really don't think I can come up with a better answer, It's unfortunate but oh well.

So the next question is what do we replace the colour with? There's the obvious answer: Grey, or black, but as grey and black are in our list of colours, what if I chose to remove one of those from existence, then what happens?

Another alternative is to have it be some random other colour, it doesn't matter what, as long as it's different.

Alternatively, we can just make it a new colour, decide that it looks different. How does it look now? Well it's impossible to say, we can only perceive the colours that we know exist. It would be a brand new never before seen colour. Unfortunately that means we can't know if it would be pretty or not, making it a bit of a gamble.

Okay I think the problem here is that I included, what I called, the "pseudo-colours". It's intuitive what it means to take away a hue. But taking away a lightness value doesn't have any intuitive explanation. So I'll amend my previous definition of a colour. Black, White, and Gray are no longer colours, we are specifically preventing the brain from processing specific hues of colours, instead replacing them with grey.

There are more possible solutions but it's nearly 11pm and I was supposed to be asleep an hour ago.

So, with this definition, of the 6 colours available to me: Red, Orange, Yellow, Green, Blue, and Purple, I would remove Orange.

It can be a pretty colour, but so can all the others, and every other colour has its own associated meaning that needs to be preserved. Orange doesn't have a meaningful identity, other than a fruit, that I don't even like.

Sorry orange.