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

Index: dwm/config.def.h
===================================================================
--- dwm/config.def.h.orig	2014-02-09 15:24:13.708117011 +0100
+++ dwm/config.def.h	2014-02-09 15:24:13.700117011 +0100
@@ -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 */
Index: dwm/dwm.c
===================================================================
--- dwm/dwm.c.orig	2014-02-09 15:24:13.708117011 +0100
+++ dwm/dwm.c	2014-02-09 15:24:13.700117011 +0100
@@ -792,7 +792,10 @@
 		detachstack(c);
 		attachstack(c);
 		grabbuttons(c, True);
-		XSetWindowBorder(dpy, c->win, scheme[SchemeSel].border->rgb);
+		if(c->isfloating)
+			XSetWindowBorder(dpy, c->win, scheme[SchemeSel].floatborder->rgb);
+		else
+			XSetWindowBorder(dpy, c->win, scheme[SchemeSel].border->rgb);
 		setfocus(c);
 	}
 	else {
@@ -1040,7 +1043,10 @@
 
 	wc.border_width = c->bw;
 	XConfigureWindow(dpy, w, CWBorderWidth, &wc);
-	XSetWindowBorder(dpy, w, scheme[SchemeNorm].border->rgb);
+	if(c->isfloating)
+		XSetWindowBorder(dpy, w, scheme[SchemeNorm].floatborder->rgb);
+	else
+		XSetWindowBorder(dpy, w, scheme[SchemeNorm].border->rgb);
 	configure(c); /* propagates border_width, if size doesn't change */
 	updatewindowtype(c);
 	updatesizehints(c);
@@ -1051,6 +1057,8 @@
 		c->isfloating = c->oldstate = trans != None || c->isfixed;
 	if(c->isfloating)
 		XRaiseWindow(dpy, c->win);
+	if(c->isfloating)
+		XSetWindowBorder(dpy, w, scheme[SchemeNorm].floatborder->rgb);
 	attach(c);
 	attachstack(c);
 	XChangeProperty(dpy, root, netatom[NetClientList], XA_WINDOW, 32, PropModeAppend,
@@ -1522,9 +1530,11 @@
 	cursor[CurResize] = drw_cur_create(drw, XC_sizing);
 	cursor[CurMove] = drw_cur_create(drw, XC_fleur);
 	/* init appearance */
+	scheme[SchemeNorm].floatborder = drw_clr_create(drw, floatnormbordercolor);
 	scheme[SchemeNorm].border = drw_clr_create(drw, normbordercolor);
 	scheme[SchemeNorm].bg = drw_clr_create(drw, normbgcolor);
 	scheme[SchemeNorm].fg = drw_clr_create(drw, normfgcolor);
+	scheme[SchemeSel].floatborder = drw_clr_create(drw, floatselbordercolor);
 	scheme[SchemeSel].border = drw_clr_create(drw, selbordercolor);
 	scheme[SchemeSel].bg = drw_clr_create(drw, selbgcolor);
 	scheme[SchemeSel].fg = drw_clr_create(drw, selfgcolor);
@@ -1641,6 +1651,10 @@
 		return;
 	selmon->sel->isfloating = !selmon->sel->isfloating || selmon->sel->isfixed;
 	if(selmon->sel->isfloating)
+		XSetWindowBorder(dpy, selmon->sel->win, scheme[SchemeSel].floatborder->rgb);
+	else
+		XSetWindowBorder(dpy, selmon->sel->win, scheme[SchemeSel].border->rgb);
+	if(selmon->sel->isfloating)
 		resize(selmon->sel, selmon->sel->x, selmon->sel->y,
 		       selmon->sel->w, selmon->sel->h, False);
 	arrange(selmon);
Index: dwm/drw.h
===================================================================
--- dwm/drw.h.orig	2014-02-09 15:24:13.708117011 +0100
+++ dwm/drw.h	2014-02-09 15:24:13.700117011 +0100
@@ -20,6 +20,7 @@
 	Clr *fg;
 	Clr *bg;
 	Clr *border;
+	Clr *floatborder;
 } ClrScheme;
 
 typedef struct {