I had a problem where the color would not change on my faces, no matter how many times I clicked on the “Apply” button. The model would remain grey.
I found the solution, which I’m sharing here, in case others encounter the same problem:
1- Click your Cylinder in the hierarchy
2- In the Inspector, look at your “Mesh Renderer” component
3- In the “Materials” section, click the little circle to change from “Lit” to “UnlitVertexColor”
4- Try the tutorial steps again and the colors should now appear! (with a yellow gradient at first, but it disappears after you add the 2nd red color)
*Edit Oct. 7: while the solution above works, you may dislike the visual that it results in (a solid color with no details and no cool shadows).
If you would like to push this further, I encourage you to look at the “Shader Graph” solution proposed by rhirne12 in this post: Changing the Color on Vertex