Change where a vehicle's color affects.

[DOUGL4S1]

I was trying to do a simple mod to make the Bus look more like a classic New York bus, white with a blue stripe:

I kinda liked the final result, however, the top of the bus also becomes blue, and it messes up the texture of one of the decals (the darker square around the one on the roof). After some research, I saw that the top of NY buses are white, not blue, and after some testing, the decal does that with darker colors, but with colors like white it stays fine:

How can I make the Bus' roof to be affected by the primary color instead of the secondary? Sorry if there's a thread about that already. And I know it's a minor thing, it just kinda bothers me.