How to color the face on the 3D file model and disable color interpolation

When I use code vertexColors: true. I found that the other faces that were not in the coloring range were also colored

I solved it by building a new geometry from the faceIndexs of the model, will there be a better way to solve this problem?

It sounds what you did is just right. When you want to ensure vertex color definitions do not affect the final color of a specific face, you have to make sure no respective vertex hold a vertex color other than 0xffffff.