in red 还是in a red

是in red 不是in a red，in red为固定搭配中文意思是穿红衣

Christmas cookies with frosting and sprinkles， and "Happy Holidays" written in red and green across the tray itself.圣诞饼干上有糖霜和糖屑，托盘上用红色和绿色写着“节日快乐”。

The route has been marked in red.

in red 还是in a red

in red 释义：

穿红衣

例句：

They walked among the crowds in Red Square.

他们走在红场上的人群之中。

in a red释义：

在一个红色的

例句：

Father Christmas dresses up in a red coat.

圣诞老人用红色外套乔装打扮。

He put his wiper control in a red metal box that on the outside had the words "For Engineering Tests Only. Do Not Open.".

他将刮水控制器放置在一个红色的金属盒子里，上面写道 “工程测试专用，切勿打开。”。