Adding @ashoat as first pass reviewer since this involves flow

Tried using a Map instead of object:
https://flow.org/try/#0MYewdgzgLgBBCmAbewrwCYBUAWAneAhugMIiIi4QDyARgFYwC8MY8A7jALIEAOAFAG0AUDBgCA5ABYAQgA4A7AEFF4gDQwAjAF1VIseICsxAJwAxA6bUwATDr0S58gwGYAIled3RE5TJlXJL30ANldJY0VpKwMgiWJZAwAGZKtg2PFk2VNZZyt5dODJWWJlK1l0g3lg43lLdWMKpyarDUS7LQBKAG4hIVBIWARkVAwcfCJSckoALhgAEgAlQnQqMEQAT0VcXAJ1gB5oXABLMABzAD4mGC2d9YA6ADNcEABbPiGUNCw8ZcmKanodwA1vB1hA+B1ur14AAPHgUWBQdY8eAwADKSE+ox+EzI-yucwA0qCIHskSiQA84JiRt9xiQ8ZRaHRzj0+uBoDACLMMcMvmNfoyIFcAEQyBTKEVdIA

Typing selectedThreadColorsObj doesn't work:

https://flow.org/try/#0MYewdgzgLgBBCmAbewrwCYBUAWAneAhugMIiIi4QDyARgFYBcMA3gNpgEC28T0uAlmADmAXSZgArpxrxcAXxgBeFgCgYMAEQAWAEIAOAOwBBIxqYBGADRrNAVgP37ZmACZrclStCRYCZKgwcfCJSckomABIAJUJ0KjBEAE8jXFwCRIAePkEhAD4lGFo6FCgAOngwKAF4CAAKGz8SwLxY0IpqemsAShtSiAooWtqCSxgaLqV85ht1fCgJXDAYAlZzERgAWjHVkQBuGzke9VLOAgAHWsJgbEmYK+xWAAYRLv2gA

Let's flip the keys/values of selectedThreadColorsObj and use $Values

Unfortunately wasn't able to get this approach to work..

$Values<T> represents the union type of all the value types (not the values, but their types!) of the enumerable properties in an Object Type T.

const selectedThreadColorsObj = {
  'a': '4B87AA',
};

export type SelectedThreadColors = $Values<typeof selectedThreadColorsObj>;

In this case type SelectedThreadColors = string

const selectedThreadColorsObj = {
  'a': '4B87AA',
  'b': 123,
};

export type SelectedThreadColors = $Values<typeof selectedThreadColorsObj>;

In this case type SelectedThreadColors = string | number

https://stackoverflow.com/questions/39154870/how-to-use-define-enums-with-flow-type-checking#comment98378846_49105296

But I'm also wondering if we really need an object... is there a way to make it working with just an array? Do we use the object values in some other way than just for ordering?

I don't think there's a way to make it work with just an array. The reason we want to use an object is so we can use the flow $Keys utility type to essentially create an enum of string literals. I don't think there's any way to create a flow type that's an enum of string literals from an array?

The alternative is to write out the string literals twice (see below). Once for the exported array of string, and once for the type which is an enum of string literals. This may be more readable, but it's less maintainable as any change in the selected colors would need to be changed twice in the code.

const selectedThreadColors: $ReadOnlyArray<string> = ['blah', 'blah2', 'blah3', 'blah4'];
export type SelectedThreadColors = 'blah' | 'blah2' | 'blah3' | 'blah4';

I've found this SO question https://stackoverflow.com/questions/54777850/flow-using-array-values-as-type where they present two possible solutions. Not sure if they're a lot cleaner, but they avoid introducing the object keys.

But to be fair, spending too much time on it doesn't make a lot of sense.