This is because their colors are not bright enough to be noticed by the human eye.

这是因为它们的颜色不够明亮，无法被人眼注意到。

youdao

Is it too bright or not bright enough?

是太亮还是不够亮呢？

youdao

The Apollo landings took place during lunar mornings so the stars were not bright enough to be captured on camera.

阿波罗登陆的时间对于月球来说是早上，所以星星没有那么明亮，相机也很难捕捉到。

youdao

She gets the sadness of the character but not the flip side of her personality, and she's just not a bright enough spark to keep Gatsby's torch burning for so long.

CNN: Luhrmann's 'Gatsby'? It's a shame

One who tried to do so late last year, a bright reformer called Koichi Kato, proved not bright enough to carry his erstwhile supporters with him, and is now in the political wilderness.

ECONOMIST: Why Japan��s Mori must go

As we've said before, though, the few enterprise-related bright spots are likely not enough to hold off the company's continued slide.

ENGADGET