Regardless of what color i am making the car body, it will come out brown. That's all that it is to it.

Steps to get the issue

1. Create a car body out of any concrete colored block
2. Put the item in the workstation
3. The car body will result brown

Versions:

• Minecraft version: 1.16.4
• Forge version: 35.0.7
• Mod version: 1.0.4

Other mods:
Ultimate Plane Mod (mod version: 1.0.1)
Just Enough Items (mod version: 7.6.0.58)

Brown is the base color of every car, just the accent color changes.
Look at the second image on curseforge.