na-adjective
Describes a vivid, clear red color. Used as an adjectival noun (na-adjective).
彼女は明赤色のドレスを着ていた。
She was wearing a bright red dress.
紅色 is a deep crimson or scarlet, often with a slightly darker or more traditional nuance than 明赤色.
Compound of 明 (bright) + 赤色 (red color). The reading めいせきしょく follows the on'yomi of the characters.