If your document uses a very old Arabic font (from Word 6 times), maybe Word considers it to be a symbol font.
With any Unicode font and Arabic text, you should be able to copy/paste text into "Find what" just fine.
Also, the color of diacritics in Arabic is a style setting, I think. So you shouldn't need to use "Find/Replace" and just edit the style.
Maybe you'll find some tool on the net to convert your old Arabic font to Unicode. I know such tools exist for many old Greek fonts.
Regards,
Klaus