Press Ctrl-H, and click the More button in the Find and Replace dialog. Click the find tab.

Click the Format button, and select Font.

Select the color to be changed—leaving all else blank—and click OK.

Click the Replace tab, and repeat steps 2 and 3 to select the new color.

Click Replace All.

Another ref.

https://cybertext.wordpress.com/2019/03/30/word-find-and-replace-a-custom-font-colour/

1. Open the Find and Replace dialog ( Ctrl+H).
2. Make sure the cursor is in the  Find field.
3. Click  More.
4. Click  Format.
5. Choose  Font.
6. Click the drop-down arrow for  Font Color.
7. Click  More Colors.
8. Enter the RGB values you want to find (e.g. 255, 51, 153).
9. Click  OK. The text under the Find field should show the RGB values you selected; for example: Font Color: Custom Color (RGB(255,51,153)).
10. Place your cursor in the  Replace field.
11. Repeat Steps 4 to 7.
12. Enter the RGB values you want to use for the replace (e.g. 230, 131, 76).
13. Click  OK. The text under the Find field should show the RGB values you selected; for example: Font Color: Custom Color (RGB(230,131,76)).
14. Click  Find Next, then click  Replace for the first one found.
15. Repeat Step 14 until all are found, or, if you are confident that you won’t mess up anything else, click  Replace All.