Datum in excel kleuren

This can be accomplished using conditional formatting:

There's two rules:

1: Red when the value in Column N is less than OR greater than 3 months of the date in column O:
Conditional formatting 3.png

2: Green when the value in Column N is greater than or equal to AND less than or equal to 3 months of the date in column O:
Conditional formatting 2.png

Make sure that both rules apply to the whole of column N:
Conditional formatting 1.png

This way the fonts will be red or green only when there is a date. When there is something other than a date (like the title of the column) then the font colour won't be affected by the conditional formatting.

You can copy the rules here:

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I used Google translate to translate your post so apologies if I've misunderstood your question.