Select an object or surface and assign it one of the numbered surface boxes in the bottom left of your interface...you do this by clicking on the numbered box.
Then, right-click on the surface box...Select
Edit Material from the dropdown menu.
That'll bring up a dialog box, where you can change the parameters and edit the color.
Diffuse will be the one next to the little box that shows the current color. Click on that + box and you can edit the Red Blue Green values and change the color.
All this is detailed in the User Manual, which I suggest you download from the main AC3D site, and if possible, print out a nice color copy for future reference. It'll save you a lot of trouble.
