How does Word choose which colour to allocate to which author?
You can choose to display tracked changed in any of 16 colours. But if you want Word to display different reviewers’ revisions in different colours (that is, if you choose to display using the “By Author” color), then Word only provides 8 colours with which to distinguish authors. In theory, Word allocates colours to reviewers in the following order: red, blue, green, violet, dark red, teal, dark yellow and grey. For the 9th and subsequent reviewers of a document, Word cycles through the colours again. But in practice, as reviewers come and go (as their revisions are accepted or rejected), the allocation of colours to reviewers ends up, in effect, random. You can test this out by changing the user’s name at Tools > Options > User Information, turning on tracked changes and making some changes to your document.
