More red; having a greater amount or intensity of the color red.
From Old English 'read,' which is shared across Germanic languages (German 'rot,' Dutch 'rood'). The comparative form 'redder' is simply the base adjective with the comparative suffix '-er' added.
Red is one of the most ancient color words in human language—most languages developed names for red before blue, probably because red objects (blood, fire, berries) were more immediately important to survival. It's why redder sounds so natural while 'bluer' feels less common.
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