Cinza vs. cinzento
In Portuguese, cinza and cinzento both translate to "grey." Although they are often used interchangeably to describe color, they have different grammatical rules and origins. Cinza is originally a noun meaning "ash," while cinzento is a descriptive adjective.
Cinza
A1Primarily meaning "ash" (the residue of fire), this word is used as the name of the color itself. In standard grammar, when used as an adjective, cinza is invariable, meaning it does not usually change form to match the gender (masculine/feminine) or number (singular/plural) of the object.
Eu comprei uma camiseta cinza.
(I bought a grey t-shirt.)
Os olhos dele são cinza claro.
(His eyes are light grey.)
Há muita cinza no chão por causa do fogo.
(There is a lot of ash on the ground because of the fire.)
Eles preferem os carros cinza.
(They prefer the grey cars.)
Cinzento
A2This is the descriptive adjective form derived from cinza. Unlike cinza, cinzento must always agree in gender and number with the noun it describes (becoming cinzenta, cinzentos, or cinzentas). It is heavily used to describe the weather, implied gloominess, or something that is dull or "ash-like."
O céu está muito cinzento hoje.
(The sky is very grey today.)
Ela usava umas calças cinzentas.
(She was wearing some grey trousers.)
Era uma manhã fria e cinzenta.
(It was a cold and grey morning.)
Os prédios pareciam velhos e cinzentos.
(The buildings looked old and grey.)
Summary
Use cinzento (and its variations) when you need a standard adjective that modifies a noun's gender and quantity, particularly when describing cloudy weather or a dull appearance. Use cinza when referring to the physical substance "ash," the abstract concept of the color, or as a fixed adjective that generally does not change plural/gender forms.