adverb, adverb taking the 'to' particle
bright red; glowing red; brightly
Describes something burning, glowing, or shining with a vivid red light; often used for fires, embers, sunsets, or flushed faces. Can also be used adverbially with と.
炭火が赤々と燃えている。
The charcoal fire is burning bright red.
夕日が空を赤々と染めた。
The setting sun dyed the sky bright red.
真っ赤 means 'bright red' or 'deep red' as a noun/adjective, often for solid red objects or embarrassment. 赤々 emphasizes a glowing, luminous red quality, often with light or fire.
Reduplication of 赤 (あか, 'red') with the iteration mark 々, forming an adverb emphasizing a vivid, glowing red state.