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 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。”。