Hi, TrueMonolith.
If I understand corectly what you want to achieve that is the same effect as you can see on the IDAP Ambulance, for instance. If you want to lower reflection values of whole vehicle then you can create custom material and play with these values:
specular[]={1,1,1,1};
specularPower=100;
if you want be pixel specific then you have to plug a SMDI map in your material which you can prepare easily in some photo editor. Then you need to edit green and blue channel to get wanted effect.
an example how we did it for the "unfinished van livery":
