When picking any manufacturer color, the decals stay the same color as if you picked the standard color. So if you pick green for example, the decals remain orange instead of turning light green as they should.
The decals won’t disappear when picking solid manufacturer colors either.
I encountered the same issue.
It happens with ALL the cars that have decals (stripes, words, patterns, etc.) : Dodge and SRT Vipers (black and white stripes), 70’s Muscle cars, Ferrari FXX K, etc.
This happens since the February update of FM 7 on Xbox One X.
Please fix this.
No fix in today’s update… 
Still no fix in today’s update…