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

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

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.

1. an illusory hope

e.g. chasing rainbows

2. an arc of colored light in the sky caused by refraction of the sun's rays by rain