Cuello vs. collar
While both collar and cuello relate to the neck area, they are not interchangeable. Cuello primarily refers to the anatomical body part or the part of a garment, whereas collar refers to an accessory worn around the neck.
Cuello
A1This word refers to the neck of a person or animal. By extension, it also means the part of a piece of clothing that goes around the neck (like a shirt collar) or the narrow, top part of a bottle.
La jirafa tiene el cuello muy largo.
(The giraffe has a very long neck.)
Me duele el cuello después de mirar el teléfono por horas.
(My neck hurts after looking at my phone for hours.)
Por favor, abróchate el botón del cuello de la camisa.
(Please, fasten the collar button on your shirt.)
Me gusta usar camisetas con cuello en V.
(I like to wear V-neck t-shirts.)
Sirve el vino sujetando la botella por el cuello.
(Serve the wine by holding the bottle by its neck.)
El tráfico en esa calle es un cuello de botella.
(The traffic on that street is a bottleneck.)
Collar
A2This word refers to an item worn around the neck for adornment or identification. It is equivalent to a necklace for people or a collar for a pet.
Mi abuela me regaló un hermoso collar de perlas.
(My grandmother gave me a beautiful pearl necklace.)
El perro necesita un collar nuevo con su placa de identificación.
(The dog needs a new collar with its ID tag.)
Compré un collar antipulgas para el gato.
(I bought a flea collar for the cat.)
Ese collar de plata combina perfectamente con tus aretes.
(That silver necklace perfectly matches your earrings.)
La reina llevaba un collar de diamantes impresionante.
(The queen wore a stunning diamond necklace.)
Summary
In short, cuello is the body part (your neck), a part of a shirt (a shirt collar), or the neck of a bottle. Collar is a separate item you wear around your neck, like a necklace or a pet's collar.