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.
           彩虹般五颜六色的绸缎

2. 五颜六色；色彩缤纷
    A rainbow of colours is a wide range of bright colours.

    e.g. ...a rainbow of coloured cushions.
           五颜六色的彩垫

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.
           找到一种治愈方法的希望——就如同永远够不到的一桶黄金——往往让理智的人们做出不理智的事情。