In general, 赤 and 紅 means red and no need to concern how red they are. 赤 is the more formal term and literary. For example, if you want to translate "The Red Queen" from Alice in Wonderland, it would be more formal to translate it as 赤后 instead of 紅色皇后. Battle of Red Cliffs would be 赤壁之戰 rather than using 紅. On the other hand, 赤花 is too formal, 紅色花 is better.