Looking good. The alpha line when dragging is a great idea.
Suggestions.
Could we have ALT as the shortcut for the colour picker?
Double clicking the header to maximise/minimise.
Ability to assign keys to right mouse, eg eraser.
Resizing brushes could resize fast if the key is held down for 1/2 a sec of so.
Warning when closing the document to save.
Holding shift to allow straight lines to be drawn.
The number of Undo/Redo is amazing. Could we get them to cycle faster if we hold down the keys?
Also we must be ready for layers right? :)