Add a rule called Paint:-

Players may have Paint, which must be specified in eir Items field as ((Color)) Paint, where ((Color)) is the color code of that Player’s Homeland Square at the time the Paint is purchased. A single use of Paint may be purchased at any time at a cost of B$10.  When a Player occupies a Square, e may use a Paint e has to change one color code of the Square e is on to the color of the Paint, which is then removed from eir Items list. That Square has then been Painted that color. Painted Squares may override the normal color coding restrictions detailed in rule 14 - World, and its subsections (for example, grey squares stay painted, warp squares may be painted, etc.).

Note: Edited the brackets to parenthesis, since using brackets (either single or double) is part of the wiki syntax.—Chronos
Edited the title to look right—Excalabur