Orange might seem more pervasive than it is because prisoners often change into orange when they go out in public. Sheriffs sometimes put prisoners in orange during perp walks in front of reporters, and prisoners often wear orange in court.
The Federal Bureau of Prisons issues khaki trousers and shirts. New York state actually bans the color orange among prisoners: It issues uniforms that are "hunter green," and lets them wear their own T-shirts, as long as they're not blue (the color of prison-guard uniforms), black (too hard to see), gray (other officials wear it), or orange (the color worn by the Correctional Emergency Response Team, or riot control).
Today, uniforms generally serve the practical purpose of providing a means to differentiate inmates from civilians and staff as well as to distinguish custody levels. In California, for example, orange jumpsuits are reserved for new inmates who haven’t yet been classified. Those in the general population wear some variation of blue, white and grey—for men, jeans or blue scrubs-like pants and shirt, as well as white t-shirts and grey sweatshirts. Women wear the same basic clothing—blue and white shirts and blue, chambray or denim pants. Inmates working on the perimeter—including those outside the prison gates—wear a green jumpsuit.
And there are even some places that allow inmates to wear their street clothes, although fewer and fewer places permit this. At Clinton Correctional Facility, from which Richard Matt and David Sweat escaped on June 6 2015, street clothes are a privilege for inmates in the honor block who haven't committed any infractions. (The honor block has since been shut down.) Given the escape, it seems likely that all New York facilities will go the route of Michigan, which permitted street clothes but changed the uniform to dark blue and orange, years ago.
Prison blues are mild compared to the lengths that some jurisdictions go to humiliate inmates through clothing. Some prisons make a point of picking distinctive uniforms. Cleveland County makes prisoners wear pink shirts and yellow-and-white striped pants, which sheriff's officials say makes escape more difficult. Sheriff Joseph Arpaio of Maricopa County, Arizona, has at various times forced prisoners to wear traditional stripes, orange jumpsuits, and pink underwear. The office of the sheriff's website says that the pink underwear is to "prevent inmates from stealing the white shorts." Arpaio also thought it was clever to put the word "ESCAPEE" on the inside of inmates' uniforms, in case they should try to wear them inside out while on the run.