In that case the simpliest solution I've found is from a thread on Mr.Excel from June 2001 where Aladin Akyurek described probably the simpliest non-"Hello World" UDF (user defined fuction) I've ever seen:
That returns a boolean, much like ISNUMBER or ISTEXT.
Anyway you conditional format cell A1 with whatever font you want using the function rule
=ISFORMULA(A1)
Then you apply it to whatever range of cells you want to check.
Hope that helps.
EDIT:
If I'm misunderstanding and you just want to reproduce the functionality of track changes with conditional formatting instead of distinguising formulas from inputs, then check out this thread on superuser.
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