Basically yes. Not sure of the usefulness of the "lock" at step3 but Gimp lets you prevent chanages to a layer (pixel/alpha/position locks).
As an option, if you drawing is black on white, you can put your "paint" layer in Darken only mode and since your colors will likely be darker than white but lighter than black the pencil drawing will show through the paint.
PS; better make a scan: 1) no need to fix geometry afterwards (you need extreme care to take a strictly rectangular photo with a phone) and 2) uniform lighting so the image is much easier to process.
As an option, if you drawing is black on white, you can put your "paint" layer in Darken only mode and since your colors will likely be darker than white but lighter than black the pencil drawing will show through the paint.
PS; better make a scan: 1) no need to fix geometry afterwards (you need extreme care to take a strictly rectangular photo with a phone) and 2) uniform lighting so the image is much easier to process.