On Step #7 I would suggest to color rectangle with completely different color from colors in your photo (then it will be easy to notice if you miss color on some mesh points).

As for me, it's better on Step #9 pick 5 colors from the original photo and assign them to the mesh points right after converting rectangle to Mesh (when your mesh is going to be more complicated than this example, you could find it's not easy to select some mesh points to assign colors)

And then when adding a mesh points, immediately assign colors to them.

Also, using a Lasso Tool (shortcut: Q) will allow you easy to select a several points (to assign a one color from the original -- there are some places where colors are the same)