Basic observation: أَحْمَرُ is an adjective and is ghayr munsarif.
Contrast: كَرِيمٌ (generous) is also an adjective, but it is fully declinable and takes tanwin.
Why? Adjectival meaning alone does not make every adjective ghayr munsarif. A second recognized cause must combine with it.
Opening interactive controls...
Causes of Ghayr Munsarif: Wasf and the Pattern Af'al
Basic observation: أَحْمَرُ is an adjective and is ghayr munsarif.
Contrast: كَرِيمٌ (generous) is also an adjective, but it is fully declinable and takes tanwin.
Why? Adjectival meaning alone does not make every adjective ghayr munsarif. A second recognized cause must combine with it.