1. Open DB_US.bnk (Found in Euro/Bnk/DataBase) with BNK File Editor, and extract TDU_CarColors.us.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color1.jpg
2. Open TDU Modding Tools, and select the Car Colors wizard.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color2.jpg
3. This window should appear.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color3.jpg
4. Load the extracted TDU_CarColors.us.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color4.jpg
5. After loading TDU_CarColors.us, this should appear.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color5.jpg
Assuming we're swapping the Farboud's blue with the Zonda's matte black;
6. Select the Farboud's blue and click {Set 1}.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color6.jpg
7. Then, select the Zonda's matte black and click {Set 2}.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color7.jpg
8. Next, click Switch and the dialog should appear.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color8.jpg
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color9.jpg
9. Now, open DB_US.bnk BNK File Editor again, and select TDU_CarColors.us again, and click Replace selected file...
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color10.jpg
10. This dialog will then appear, click OK.
http://i103.photobucket.com/albums/m131/suakukia/Etcs/color11.jpg
11. After this is done, click Save BNK.
Restart your game, and the colors should be swapped. :)