Put your data into an array then use flatMap to unwrap each .items:

[greenData, redData, blueData].flatMap(d => d.items ?? [])
//=> [ {id: 36, revision: 1, updatedAt: '2021-09-10T14:31:07.164368Z'}
//=> , {id: 35, revision: 1, updatedAt: '2021-09-10T14:29:54.595012Z'}]

If you fancy you could abstract d => d.items ?? [] with a bit of curry (no pun intended ;)

const take = k => o => o[k] ?? [];

Which gives us:

[greenData, redData, blueData].flatMap(take('items'))

We can even go a step further if you ever need to repeat this process with different keys:

const concatBy = fn => xs => xs.flatMap(x => fn(x));

Now it almost feels like you're expressing your intent with words instead of code:

const takeItems = concatBy(take('items'));

takeItems([greenData, redData, blueData]);
//=> [ {id: 36, revision: 1, updatedAt: '2021-09-10T14:31:07.164368Z'}
//=> , {id: 35, revision: 1, updatedAt: '2021-09-

Let's build another function:

const takeFood = concatBy(take('food'));

takeFood([{food: ['🥑', '🥕']}, {food: ['🌽', '🥦']}]);
//=> ['🥑', '🥕', '🌽', '🥦']

Addendum

This is only meant as a potentially useful learning material. My advice is to use flatMap.

This:

[[1, 2], [3, 4]].flatMap(x => x)
//=> [1, 2, 3, 4]

Can also be expressed with reduce. Slightly more verbose but does what it says on the tin:

[[1, 2], [3, 4]].reduce((xs, x) => xs.concat(x), [])
//=> [1, 2, 3, 4]

So to put it simply you could also do:

[greenData, redData, blueData].reduce((xs, x) => xs.concat(x.items ?? []), [])

You can do this using the Logical OR operator which lets you provide a default value if the items field is missing.

const blueData = { items: [ { id: 35, revision: 1, updatedAt: '2021-09-10T14:29:54.595012Z', }, ], };
const redData = {};
const greenData = { items: [ { id: 36, revision: 1, updatedAt: '2021-09-10T14:31:07.164368Z', }, ], };

const colorData = [
  ...(blueData.items || []),
  ...(redData.items || []),
  ...(greenData.items || []),
];

console.log(colorData);

Maybe I'm a little old-fashioned but I'd use concat for that:

The concat() method is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/concat

const blueData = {
    "items": [
        {
            "id": 35,
            "revision": 1,
            "updatedAt": "2021-09-10T14:29:54.595012Z",
        },
    ]
}

const redData = {}

const greenData = {
    "items": [
        {
            "id": 36,
            "revision": 1,
            "updatedAt": "2021-09-10T14:31:07.164368Z",
        }
    ]
}


const colorData = [].concat(blueData.items,redData.items,greenData.items).filter(x => x)

console.log(colorData)

the last filter is for removing undefined values