Single-Colored Portal Guns are tools added by the Portal Gun Mod. They fire only one type of portal, and the color varies depending on the type you have. Also, like the dual-colored Portal Gun, they will automatically merge with the single coloured portal gun of the other colour to form a dual portal gun. They can also be crafted together.
While not efficient for survival, they are commonly used in adventure maps that use the Portal Gun Mod. In these cases, you usually start off with a Single-Colored Portal Gun (color may vary with the map). You can then use this gun to teleport to locations where a Portal Spawner of the corresponding color (blue/orange, red/yellow, etc.) is.