Monochrome just means "a painting, drawing, or photograph in a single hue." But I think their intent was trying to describe an absolute minimum editor. Think Microsoft paint with the pencil tool but it only draws a single pixel per click.

Their list of things is fairly reasonable and will result in a fairly long main quests arc. Example. Depending on your familiarity with these items, as well as your familiarity with rust, it could keep you busy for a while in a non-throwaway way.

I wonder if part of the reason you're running into a wall is because you're asking what could be one of many different questions and haven't provided many hints to help infer intent.

For example, you could be asking "how do I write a Rust application with a GUI if Rust doesn't provide out of the box GUI functionality?" In the case you'll need to use one of the many GUI libraries that are out there. These will let you do screen layout, add buttons, etc.

You might also be asking, "am I going to have to build my own UI elements, like buttons and menus, from scratch?". The answer to this is no, unless you want some kind of new/novel UI element. For example, I made a checked list box item in C# about 8 years ago. All the GUI libraries will provide mechanisms to declare and position things like menus and buttons. They'll also provide hooks to let your application know the user did a thing.

Finally, you might be asking about how to actually draw/render the picture portion of your graphics editor. The GUI library you choose will significantly impact this. Example. Example. Example. Note that these may or may not be good choices to use.