I am attempting to color the walls of different systems with different colors using the -attr flag and a user defined wall: portion of code in scrap: line u:wall -attr color "(0.1, 0.2, 0.8);" 1392.9359853260457 -71.62435442682576 1395.5759853260456 125.20564557317425 endline portion of code in layout: def l_u_wall (expr P)= T:=identity; if known(ATTR__color): pickup PenA; draw P withpen PenA withcolor ATTR__color\n fi; enddef; initsymbol(\"l_u_wall\"); I am not having any success however. Is it possible to set and read attributes from lines as I am trying to do? Is somebody aware of an alternate way to pass in a color argument to the metapost definition (instead of defining separate walls for each different colored system)? Thanks, Carl