(as|) different as chalk and cheese

not in any way alike in character or quality

The origin of the idiom "different as chalk and cheese" dates back to the notable contrast between two distinct substances: chalk and cheese. Chalk is a soft, white, powdery substance, while cheese is a solid, often yellow or orange, food product. The phrase is used figuratively to expresses a difference or dissimilarity between two things, emphasizing their contrasting nature.

Their personalities are different as chalk and cheese; one is outgoing and extroverted, while the other is introverted and reserved.
The two paintings displayed at the gallery are as different as chalk and cheese; one is abstract and vibrant, while the other is realistic and muted.
The siblings have completely different interests and hobbies; they are different as chalk and cheese.
The two candidates' policy proposals are as different as chalk and cheese; one focuses on increasing taxes, while the other emphasizes tax cuts.
Despite being siblings, their fashion styles are as different as chalk and cheese; one prefers casual and sporty attire, while the other opts for elegant and formal clothing.