By "wireframe" do you mean:
[ol 1]
[li]you have solid/sheet bodies, and have switched to a wireframe rendering mode and want to change the color of the body edges[/li]
[li]you have created a network of curves (sometimes referred to as "wires") and want to change their color[/li]
[li]you have solid/sheet bodies in a shaded rendering mode and want to change the color of their edges[/li]
[li]none of the above[/li]
[/ol]
For case 1, the edges will take on the body color, which can be changed through Edit -> object display (as noted by jerry1423).
For case 2, if the curves are part of a sketch they will default to the sketch color preferences, though there is an option to turn this off and display them as a user specified color instead. This can be found in Preferences -> sketch -> sketch settings -> display object color (note that if you turn this option on, you will lose the color coding of the sketch status: red for overconstrained, green for fully constrained, tan for reference curves, etc). If they are not part of a sketch, you can change their colors using Edit -> object display.
For case 3, you can change the edge color used in the "shaded with edges" rendering mode by going to Preferences -> Visualization -> visual -> general display settings -> shaded edge color.
