Since you refer to "the color of spelling and grammar errors" it does appear as if you are talking about spellcheck... This can't be customized, I'm afraid.

In the Track Changes Options dialog box (Review tab | Track Changes | Change Tracking Options), what you can do is choose from a selection of colors for insertions, deletions, comments, etc. (applies to Word 2007 and 2010).

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Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional