"The dress is black."
Translation:La robe est noire.
Because in french, the adjective agrees in number and in gender with the noun. So, since "dress" is feminine, you have to add "e" at the end of the word.
For example, "La robe est noire", "le stylo est noir", "les voitures sont noires" (plural).
There are many exceptions for the color adjectives, the rule is pretty complicated... The noun and the adjective agree only if the color exists only as an adjective, not a noun. For example "orange" is a fruit, but also a color. "Marron" too (chestnut/brown). In this case, the adjective does not agree (la robe est orange, les voitures sont orange (without "s")). BUT... This rule has also some exceptions ! "Rose" is a color (pink) and a flower, but it agrees with the noun. But don't worry, it doesn't matter a lot, a lot of french people makes mistakes with these adjectives of colors :)