I tried to create a skin using the GT86 Template and this color but the car would always take the color of the metaldetail.dds file
The skin system in AC is built with multiple different image files that control different things. Any file you have in your livery folder will be used instead of that car's default texture
for that livery only. At the basic level there's the main diffuse file (which is where you paint a complex livery). If this file has a black alpha channel, it will be blended with the metaldetail.dds file. The detail file controls the whole car colour (in the case of solid colour cars, so they can all share the common diffuse file reducing filesize) as well as some of the advanced effects like the metallic flake.
If you do not want to use those effects, make sure the alpha channel of your livery files is
white. That makes the livery 'opaque,' for lack of a better term, preventing it from being blended with the metaldetail.dds file.
As mentioned, if you are not making use of the metaldetail file, you do not need to put it in your livery's folder - the game will simply use the default file instead. Note that just deleting the file is
not always a solution to the issue you are having - to prevent the livery being blended with the metaldetail file you
must make your alpha channel white. Some cars (several of the Ferraris, for example) have a default metaldetail file that is red. If you do not have a metaldetail file and your livery alpha channel is black (or grey) it will be blended with the default red detail file.
The system sounds complex at first, but really it's quite clever and provides a lot of flexibility. If you want to get into some of the more advanced aspects of how the system works to create different surface finishes on your liveries,
have a look at this post by Val_N on the official forums. It describes what the metaldetail.dds file does as well as the '_map.dds' file.