To reproduce:
1. Open the project and TestScene in it
2. In the sample, select Canvas > Custom Mesh.
3. See the mesh data in the inspector. Note the vertex colors.
4. Play the scene.
5. Notice how the colors are incorrect.

This is even more noticeable if you change all colors to one, like white or red

Reproduced in 5.3.5f1, 5.4.0f3, 5.4.3p2, 5.6.0a6

Workaround: Use Color32 instead of Color