URL: http://dwm.suckless.org/patches/historical/float_border_color
float_border_color2 allows you to specify a different border color for floating
windows.

diff -r ec4baab78314 config.def.h
--- a/config.def.h	Mon Dec 19 15:38:30 2011 +0100
+++ b/config.def.h	Fri Apr 06 08:23:26 2012 +0200
@@ -8,6 +8,8 @@
 static const char selbordercolor[]  = "#005577";
 static const char selbgcolor[]      = "#005577";
 static const char selfgcolor[]      = "#eeeeee";
+static const char floatnormbordercolor[] = "#005577";
+static const char floatselbordercolor[]  = "#005577";
 static const unsigned int borderpx  = 1;        /* border pixel of windows */
 static const unsigned int snap      = 32;       /* snap pixel */
 static const Bool showbar           = True;     /* False means no bar */
diff -r ec4baab78314 dwm.c
--- a/dwm.c	Mon Dec 19 15:38:30 2011 +0100
+++ b/dwm.c	Fri Apr 06 08:23:26 2012 +0200
@@ -57,7 +57,7 @@
 
 /* enums */
 enum { CurNormal, CurResize, CurMove, CurLast };        /* cursor */
-enum { ColBorder, ColFG, ColBG, ColLast };              /* color */
+enum { ColBorder, ColFG, ColBG, ColBorderFloat, ColLast };              /* color */
 enum { NetSupported, NetWMName, NetWMState,
        NetWMFullscreen, NetActiveWindow, NetWMWindowType,
        NetWMWindowTypeDialog, NetLast };     /* EWMH atoms */
@@ -855,7 +855,10 @@
 		detachstack(c);
 		attachstack(c);
 		grabbuttons(c, True);
-		XSetWindowBorder(dpy, c->win, dc.sel[ColBorder]);
+		if(c->isfloating)
+			XSetWindowBorder(dpy, c->win, dc.sel[ColBorderFloat]);
+		else
+			XSetWindowBorder(dpy, c->win, dc.sel[ColBorder]);
 		setfocus(c);
 	}
 	else
@@ -1155,6 +1158,8 @@
 		c->isfloating = c->oldstate = trans != None || c->isfixed;
 	if(c->isfloating)
 		XRaiseWindow(dpy, c->win);
+	if(c->isfloating)
+		XSetWindowBorder(dpy, w, dc.norm[ColBorderFloat]);
 	attach(c);
 	attachstack(c);
 	XMoveResizeWindow(dpy, c->win, c->x + 2 * sw, c->y, c->w, c->h); /* some windows require this */
@@ -1615,9 +1620,11 @@
 	dc.norm[ColBorder] = getcolor(normbordercolor);
 	dc.norm[ColBG] = getcolor(normbgcolor);
 	dc.norm[ColFG] = getcolor(normfgcolor);
+	dc.norm[ColBorderFloat] = getcolor(floatnormbordercolor);
 	dc.sel[ColBorder] = getcolor(selbordercolor);
 	dc.sel[ColBG] = getcolor(selbgcolor);
 	dc.sel[ColFG] = getcolor(selfgcolor);
+	dc.sel[ColBorderFloat] = getcolor(floatselbordercolor);
 	dc.drawable = XCreatePixmap(dpy, root, DisplayWidth(dpy, screen), bh, DefaultDepth(dpy, screen));
 	dc.gc = XCreateGC(dpy, root, 0, NULL);
 	XSetLineAttributes(dpy, dc.gc, 1, LineSolid, CapButt, JoinMiter);
@@ -1741,6 +1748,10 @@
 		return;
 	selmon->sel->isfloating = !selmon->sel->isfloating || selmon->sel->isfixed;
 	if(selmon->sel->isfloating)
+		XSetWindowBorder(dpy, selmon->sel->win, dc.sel[ColBorderFloat]);
+	else
+		XSetWindowBorder(dpy, selmon->sel->win, dc.sel[ColBorder]);
+	if(selmon->sel->isfloating)
 		resize(selmon->sel, selmon->sel->x, selmon->sel->y,
 		       selmon->sel->w, selmon->sel->h, False);
 	arrange(selmon);
@@ -1776,7 +1787,10 @@
 	if(!c)
 		return;
 	grabbuttons(c, False);
-	XSetWindowBorder(dpy, c->win, dc.norm[ColBorder]);
+	if(c->isfloating)
+		XSetWindowBorder(dpy, c->win, dc.norm[ColBorderFloat]);
+	else
+		XSetWindowBorder(dpy, c->win, dc.norm[ColBorder]);
 	if(setfocus)
 		XSetInputFocus(dpy, root, RevertToPointerRoot, CurrentTime);
 }