Step 1: Understand what a Red-Black Tree is. It is a type of binary search tree with specific properties.
Step 2: Learn the properties of Red-Black Trees. They include: 1) Every node is either red or black, 2) The root is always black, 3) Red nodes cannot have red children (no two reds in a row), 4) Every path from a node to its descendant NIL nodes must have the same number of black nodes.
Step 3: Identify the leaves of a Red-Black Tree. The leaves are the NIL nodes, which are the endpoints of the tree.
Step 4: Check the color of the leaves (NIL nodes). In Red-Black Trees, all NIL nodes are black.
Step 5: Conclude that the statement 'all leaves (NIL nodes) are black' is true, and therefore the property that is NOT true is that they can be red.
