There is a brief description of the color of his helmet in the first chapter which states,
“ He hung up his black beetle-colored helmet and shined it; he hung his flameproof jacket neatly.”
This is really the only reference to the color of his uniform; however, we would assume that because the helmet is black, that the uniform would be black also. In addition to this, the uniform has two symbols on it which would probably be some type of contrasting color – these symbols would be those of the phoenix and the salamander.
I don't believe we are told directly what color the uniforms worn by the firemen were, but "black" is the logical answer to that question. We are told in the opening paragraphs of the story how the igniter set the books on fire and lit up the sky in colors of red, yellow, and black. Then Montag is described as being "burnt-corked" in color from the burning. We are told his helmet is black. There is a great deal of description of things that are black. In the next few paragraphs, Montag meets Clarisse who is dressed in white - a description meant to serve as a complete opposite to Montag. One of Montag's fellow firemen is even named "Black". Since, in this story, black is meant to represent the burned up remains of books and destruction, it is safe to assume that is the color of the uniforms.