show one's true color 直译过来就是：“露出……真正的颜色”。据说当年英国舰队航于海上时，为避开法国逡巡船队的注意，就升起法国旗帜伪装是自家人，等到关键时刻就改旗易色，显露本色。于是产生了show one's true colors 这个习语，意为“亮出自己的真正旗帜，原形毕露，显露真面目”。因此，当美国人说"She is showing her true colors."时，他/她要表达的意思就是:"She is unmasked."、"She is showing the cloven hoof."。
Benjamin: She is always speaking ill of her partner.
Terry: Yeah, she is showing her true color. She is totally a wolf in sheep's clothing.
①I was stupid enough to believe her.
②I felt the deepest respect for her skill at dissembling.
③We should draw a lesson here: do not trust appearances; they are often false.
④Her seeming friendliness was merely a disguise.
1. show the cloven hoof 原形毕露
Shortly after they became man and wife Tom began to show the cloven hoof.
I thought he would show the cloven hoof sooner or later.
2. dissemble v. 假装，掩饰
She tried to dissemble her anger with a smile on her face.
I tried to dissemble an interest I didn't feel.
Henry was not slow to dissemble when it served his purposes.
3. disguise n. 伪装，假装
She's adopted so many disguises her own mother wouldn't recognize her.
The supposed prince was really a beggar in disguise.
Not many people managed to penetrate my disguise.