This is the 1956th most frequent Portuguese word.
preta
"Preta" means "black" (feminine) in English.
The word 'preta' is serving as an adjective describing the color of the backpack, agreeing in gender and number with 'mochila', which is feminine singular.
Minha mochila é preta.
My backpack is black.
Here, 'preta' describes the t-shirt, which is also feminine singular, showing the color.
Comprei a camiseta preta da vitrine.
I bought the black t-shirt from the window display.
In this sentence, 'preta' is again an adjective that modifies 'tinta', a feminine singular noun, indicating the color in a descriptive context.