Chrome can already be done with TDUMT 1.14, it can be added to any car's Colors selection.
You can't do Gold though.
You'll need the 1.68a Community Patch installed, and TDU Modding Tools 1.14 to do this.
Start TDUMT 1.14, go to Vehicle Manager, then open up the car you want the chrome added to, and go to the Colors tab.
In the upper right corner you'll see the section called Exterior Color Sets, click the Add button in that box. A new box will pop up, add the color called Polished Aluminum. Now in the drop-down box, choose the new color name.
Now, just below that there's 2 choices, Main Color, and Secondary, that will say "Unknown color(??)" and "unavailable" next to each one. Click on the Unknown color(??) option and a new pop-up box will show up, choose the color called Silver (GR1_02). Now click on the unavailable option, and choose the color called Black (N_01). Click the big Save button in the upper right corner then start up TDU.
Go to any NORMAL paint shop and choose the new Chrome color. If you add the chrome color to any car that can't normally be painted, the only way to get that color is to buy the car again. If you can't buy it again because it's a special car, you're out of luck.