墨に染まれば黒くなる（すみにそまればくろくなる）

Japanese vocabulary

Meaning: people are formed by their surroundings; walk with the wise and become wise; associate with fools and get in trouble; that which is dyed with ink becomes black

See it used in real works

• Example sentences from Japanese novels & web fiction (Massif)
• Hear “墨に染まれば黒くなる” pronounced in real videos (YouGlish)