You become like the company you keep; associating with certain people or environments influences your behavior or character.
大学で熱心な先輩たちに囲まれて勉強するようになった。朱に交われば赤くなるだね。
Surrounded by dedicated seniors at university, I started studying more. It really is 'you are influenced by your company.'
悪い仲間とばかり付き合っていると、いつのまにか悪習に染まる。朱に交われば赤くなるから気を付けて。
If you only hang out with the wrong crowd, you'll pick up bad habits. Be careful — you become like those you associate with.
職場の雰囲気が真面目だと、自然と自分も仕事に真剣になる。朱に交われば赤くなるということだ。
When the workplace atmosphere is serious, you naturally become more professional. It shows that you take on traits of those around you.