1. 虹；彩虹
    A rainbow is an arch of different colours that you can sometimes see in the sky when it is raining.

    e.g. Oh look, a rainbow!
           看哪，彩虹！
    e.g. ...silk brocade of every colour of the rainbow.
           彩虹般五顏六色的綢緞

2. 五顏六色；色彩繽紛
    A rainbow of colours is a wide range of bright colours.

    e.g. ...a rainbow of coloured cushions.
           五顏六色的彩墊

3. 可望而不可即
    If you say that something is at the end of the rainbow, you mean that people want it but it is almost impossible to obtain or achieve.

    e.g. The promise of a cure — the pot of gold at the end of the rainbow — often makes sensible people do irrational things.
           找到一種治愈方法的希望——就如同永遠夠不到的一桶黃金——往往讓理智的人們做出不理智的事情。