因为 against 在此有两个意思,除了字面上"紧靠着"的意思 ,还有与 contrast (对立面；反差；对比)有一样的意思.
I think the bright yellow flowers look more beautiful against the blue sky.
我觉得亮黄色的花朵紧靠着蓝天看起来更漂亮.
I think the bright yellow flowers look more beautiful against the blue sky.
我觉得亮黄色的花朵对照著蓝天看起来更漂亮.
以上因为两个颜色"紧靠着",使视觉上有反差(比对) 而觉得漂亮的意思.