I would like to see a third row or fourth row of preset colors. ex. orange and purple. I would also like these presets to be locked from being changed. They would be a quick color change. Then I would want a row or two with user custom colors that can be changed at will and locked if they don't want to overwrite a favorite color.
The last change is using the middle mouse button. When I first played this I was using a wireless keyboard with built in mouse pad with no middle mouse button. This was later remedied on my end, but it did get me thinking. Assign a keyboard shortcut to enter a painting mode. I know there is a paint gun mod, but I haven't used it yet to see how it works.
Here is a rough idea of how the paint mode would work. The menu is up at all times and to select anything you need to hold alt to move the mouse cursor, as moving your view would be the default mouse movement. Left clicking would paint targeted block and right click would apply skin. You could do a scroll option that would provide a visual of areas that would be effected. ex. one block, 3x3 area, 5x5 area, etc.