Click a hex touching the pattern to turn it red;
click an edge to turn both hexes red, or a corner for all three.
Every neighbor of a red turns purple and can't be changed.
Click reds to remove them (but not the last one); purples that no longer touch a red go away too.
The Input menu offers bigger stamps; the arrow marks the hex that sits under the cursor.
Where a stamp can't be placed, clicking removes the reds under it.
The Paint menu's Alt colors only tint empty hexes and never change the pattern.
Drag to pan, scroll to zoom.